Patchwork [v7,12/27] KVM: arm64/sve: System register context switch and access support

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Submitter Dave Martin
Date March 29, 2019, 1 p.m.
Message ID <1553864452-15080-13-git-send-email-Dave.Martin@arm.com>
Download mbox | patch
Permalink /patch/761157/
State New
Headers show

Comments

Dave Martin - March 29, 2019, 1 p.m.
This patch adds the necessary support for context switching ZCR_EL1
for each vcpu.

ZCR_EL1 is trapped alongside the FPSIMD/SVE registers, so it makes
sense for it to be handled as part of the guest FPSIMD/SVE context
for context switch purposes instead of handling it as a general
system register.  This means that it can be switched in lazily at
the appropriate time.  No effort is made to track host context for
this register, since SVE requires VHE: thus the hosts's value for
this register lives permanently in ZCR_EL2 and does not alias the
guest's value at any time.

The Hyp switch and fpsimd context handling code is extended
appropriately.

Accessors are added in sys_regs.c to expose the SVE system
registers and ID register fields.  Because these need to be
conditionally visible based on the guest configuration, they are
implemented separately for now rather than by use of the generic
system register helpers.  This may be abstracted better later on
when/if there are more features requiring this model.

ID_AA64ZFR0_EL1 is RO-RAZ for MRS/MSR when SVE is disabled for the
guest, but for compatibility with non-SVE aware KVM implementations
the register should not be enumerated at all for KVM_GET_REG_LIST
in this case.  For consistency we also reject ioctl access to the
register.  This ensures that a non-SVE-enabled guest looks the same
to userspace, irrespective of whether the kernel KVM implementation
supports SVE.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>

---

Changes since v5:

 * Port to the renamed visibility() framework.

 * Swap visiblity() helpers so that they appear by the relevant accessor
   functions.

 * [Julien Grall] With the visibility() checks, {get,set}_zcr_el1()
   degenerate to doing exactly what the common code does, so drop them.

   The ID_AA64ZFR0_EL1 handlers are still needed to provide contitional
   RAZ behaviour.  This could be moved to the common code too, but since
   this is a one-off case I don't do this for now.  We can address this
   later if other regs need to follow the same pattern.

 * [Julien Thierry] Reset ZCR_EL1 to a fixed value using reset_val
   instead of using relying on reset_unknown() honouring set bits in val
   as RES0.

   Most of the bits in ZCR_EL1 are RES0 anyway, and many implementations
   of SVE will support larger vectors than 128 bits, so 0 seems as good
   a value as any to expose guests that forget to initialise this
   register properly.
---
 arch/arm64/include/asm/kvm_host.h |  1 +
 arch/arm64/include/asm/sysreg.h   |  3 ++
 arch/arm64/kvm/fpsimd.c           |  9 ++++-
 arch/arm64/kvm/hyp/switch.c       |  3 ++
 arch/arm64/kvm/sys_regs.c         | 83 ++++++++++++++++++++++++++++++++++++---
 5 files changed, 93 insertions(+), 6 deletions(-)
Andrew Jones - April 3, 2019, 7:39 p.m.
On Fri, Mar 29, 2019 at 01:00:37PM +0000, Dave Martin wrote:
> This patch adds the necessary support for context switching ZCR_EL1
> for each vcpu.
> 
> ZCR_EL1 is trapped alongside the FPSIMD/SVE registers, so it makes
> sense for it to be handled as part of the guest FPSIMD/SVE context
> for context switch purposes instead of handling it as a general
> system register.  This means that it can be switched in lazily at
> the appropriate time.  No effort is made to track host context for
> this register, since SVE requires VHE: thus the hosts's value for
> this register lives permanently in ZCR_EL2 and does not alias the
> guest's value at any time.
> 
> The Hyp switch and fpsimd context handling code is extended
> appropriately.
> 
> Accessors are added in sys_regs.c to expose the SVE system
> registers and ID register fields.  Because these need to be
> conditionally visible based on the guest configuration, they are
> implemented separately for now rather than by use of the generic
> system register helpers.  This may be abstracted better later on
> when/if there are more features requiring this model.
> 
> ID_AA64ZFR0_EL1 is RO-RAZ for MRS/MSR when SVE is disabled for the
> guest, but for compatibility with non-SVE aware KVM implementations
> the register should not be enumerated at all for KVM_GET_REG_LIST
> in this case.  For consistency we also reject ioctl access to the
> register.  This ensures that a non-SVE-enabled guest looks the same
> to userspace, irrespective of whether the kernel KVM implementation
> supports SVE.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> 
> ---
> 
> Changes since v5:
> 
>  * Port to the renamed visibility() framework.
> 
>  * Swap visiblity() helpers so that they appear by the relevant accessor
>    functions.
> 
>  * [Julien Grall] With the visibility() checks, {get,set}_zcr_el1()
>    degenerate to doing exactly what the common code does, so drop them.
> 
>    The ID_AA64ZFR0_EL1 handlers are still needed to provide contitional
>    RAZ behaviour.  This could be moved to the common code too, but since
>    this is a one-off case I don't do this for now.  We can address this
>    later if other regs need to follow the same pattern.
> 
>  * [Julien Thierry] Reset ZCR_EL1 to a fixed value using reset_val
>    instead of using relying on reset_unknown() honouring set bits in val
>    as RES0.
> 
>    Most of the bits in ZCR_EL1 are RES0 anyway, and many implementations
>    of SVE will support larger vectors than 128 bits, so 0 seems as good
>    a value as any to expose guests that forget to initialise this
>    register properly.
> ---
>  arch/arm64/include/asm/kvm_host.h |  1 +
>  arch/arm64/include/asm/sysreg.h   |  3 ++
>  arch/arm64/kvm/fpsimd.c           |  9 ++++-
>  arch/arm64/kvm/hyp/switch.c       |  3 ++
>  arch/arm64/kvm/sys_regs.c         | 83 ++++++++++++++++++++++++++++++++++++---
>  5 files changed, 93 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index ad4f7f0..22cf484 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -121,6 +121,7 @@ enum vcpu_sysreg {
>  	SCTLR_EL1,	/* System Control Register */
>  	ACTLR_EL1,	/* Auxiliary Control Register */
>  	CPACR_EL1,	/* Coprocessor Access Control */
> +	ZCR_EL1,	/* SVE Control */
>  	TTBR0_EL1,	/* Translation Table Base Register 0 */
>  	TTBR1_EL1,	/* Translation Table Base Register 1 */
>  	TCR_EL1,	/* Translation Control Register */
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 5b267de..4d6262d 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -454,6 +454,9 @@
>  #define SYS_ICH_LR14_EL2		__SYS__LR8_EL2(6)
>  #define SYS_ICH_LR15_EL2		__SYS__LR8_EL2(7)
>  
> +/* VHE encodings for architectural EL0/1 system registers */
> +#define SYS_ZCR_EL12			sys_reg(3, 5, 1, 2, 0)
> +
>  /* Common SCTLR_ELx flags. */
>  #define SCTLR_ELx_DSSBS	(_BITUL(44))
>  #define SCTLR_ELx_ENIA	(_BITUL(31))
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index 1cf4f02..7053bf4 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -103,14 +103,21 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
>  void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
>  {
>  	unsigned long flags;
> +	bool host_has_sve = system_supports_sve();
> +	bool guest_has_sve = vcpu_has_sve(vcpu);
>  
>  	local_irq_save(flags);
>  
>  	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
> +		u64 *guest_zcr = &vcpu->arch.ctxt.sys_regs[ZCR_EL1];
> +
>  		/* Clean guest FP state to memory and invalidate cpu view */
>  		fpsimd_save();
>  		fpsimd_flush_cpu_state();
> -	} else if (system_supports_sve()) {
> +
> +		if (guest_has_sve)
> +			*guest_zcr = read_sysreg_s(SYS_ZCR_EL12);
> +	} else if (host_has_sve) {
>  		/*
>  		 * The FPSIMD/SVE state in the CPU has not been touched, and we
>  		 * have SVE (and VHE): CPACR_EL1 (alias CPTR_EL2) has been
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 3563fe6..9d46066 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -351,6 +351,9 @@ static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu)
>  
>  	__fpsimd_restore_state(&vcpu->arch.ctxt.gp_regs.fp_regs);
>  
> +	if (vcpu_has_sve(vcpu))
> +		write_sysreg_s(vcpu->arch.ctxt.sys_regs[ZCR_EL1], SYS_ZCR_EL12);
> +
>  	/* Skip restoring fpexc32 for AArch64 guests */
>  	if (!(read_sysreg(hcr_el2) & HCR_RW))
>  		write_sysreg(vcpu->arch.ctxt.sys_regs[FPEXC32_EL2],
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c86a7b0..09e9b06 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1051,10 +1051,7 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>  			 (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
>  	u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
>  
> -	if (id == SYS_ID_AA64PFR0_EL1) {
> -		if (val & (0xfUL << ID_AA64PFR0_SVE_SHIFT))
> -			kvm_debug("SVE unsupported for guests, suppressing\n");
> -
> +	if (id == SYS_ID_AA64PFR0_EL1 && !vcpu_has_sve(vcpu)) {
>  		val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
>  	} else if (id == SYS_ID_AA64ISAR1_EL1) {
>  		const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> @@ -1101,6 +1098,81 @@ static int reg_from_user(u64 *val, const void __user *uaddr, u64 id);
>  static int reg_to_user(void __user *uaddr, const u64 *val, u64 id);
>  static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
>  
> +/* Visibility overrides for SVE-specific control registers */
> +static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
> +				   const struct sys_reg_desc *rd)
> +{
> +	if (vcpu_has_sve(vcpu))
> +		return 0;
> +
> +	return REG_HIDDEN_USER | REG_HIDDEN_GUEST;
> +}
> +
> +/* Visibility overrides for SVE-specific ID registers */
> +static unsigned int sve_id_visibility(const struct kvm_vcpu *vcpu,
> +				      const struct sys_reg_desc *rd)
> +{
> +	if (vcpu_has_sve(vcpu))
> +		return 0;
> +
> +	return REG_HIDDEN_USER;
> +}
> +
> +/* Generate the emulated ID_AA64ZFR0_EL1 value exposed to the guest */
> +static u64 guest_id_aa64zfr0_el1(const struct kvm_vcpu *vcpu)
> +{
> +	if (!vcpu_has_sve(vcpu))
> +		return 0;
> +
> +	return read_sanitised_ftr_reg(SYS_ID_AA64ZFR0_EL1);
> +}
> +
> +static bool access_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> +				   struct sys_reg_params *p,
> +				   const struct sys_reg_desc *rd)
> +{
> +	if (p->is_write)
> +		return write_to_read_only(vcpu, p, rd);
> +
> +	p->regval = guest_id_aa64zfr0_el1(vcpu);
> +	return true;
> +}
> +
> +static int get_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> +		const struct sys_reg_desc *rd,
> +		const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +	u64 val;
> +
> +	if (!vcpu_has_sve(vcpu))
> +		return -ENOENT;

This shouldn't be necessary. The visibility check in
kvm_arm_sys_reg_get_reg already covers it.

> +
> +	val = guest_id_aa64zfr0_el1(vcpu);
> +	return reg_to_user(uaddr, &val, reg->id);
> +}
> +
> +static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> +		const struct sys_reg_desc *rd,
> +		const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +	const u64 id = sys_reg_to_index(rd);
> +	int err;
> +	u64 val;
> +
> +	if (!vcpu_has_sve(vcpu))
> +		return -ENOENT;

Also not necessary.

> +
> +	err = reg_from_user(&val, uaddr, id);
> +	if (err)
> +		return err;
> +
> +	/* This is what we mean by invariant: you can't change it. */
> +	if (val != guest_id_aa64zfr0_el1(vcpu))
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
>  /*
>   * cpufeature ID register user accessors
>   *
> @@ -1346,7 +1418,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	ID_SANITISED(ID_AA64PFR1_EL1),
>  	ID_UNALLOCATED(4,2),
>  	ID_UNALLOCATED(4,3),
> -	ID_UNALLOCATED(4,4),
> +	{ SYS_DESC(SYS_ID_AA64ZFR0_EL1), access_id_aa64zfr0_el1, .get_user = get_id_aa64zfr0_el1, .set_user = set_id_aa64zfr0_el1, .visibility = sve_id_visibility },
>  	ID_UNALLOCATED(4,5),
>  	ID_UNALLOCATED(4,6),
>  	ID_UNALLOCATED(4,7),
> @@ -1383,6 +1455,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  
>  	{ SYS_DESC(SYS_SCTLR_EL1), access_vm_reg, reset_val, SCTLR_EL1, 0x00C50078 },
>  	{ SYS_DESC(SYS_CPACR_EL1), NULL, reset_val, CPACR_EL1, 0 },
> +	{ SYS_DESC(SYS_ZCR_EL1), NULL, reset_val, ZCR_EL1, 0, .visibility = sve_visibility },
>  	{ SYS_DESC(SYS_TTBR0_EL1), access_vm_reg, reset_unknown, TTBR0_EL1 },
>  	{ SYS_DESC(SYS_TTBR1_EL1), access_vm_reg, reset_unknown, TTBR1_EL1 },
>  	{ SYS_DESC(SYS_TCR_EL1), access_vm_reg, reset_val, TCR_EL1, 0 },

Mixing designated and non-designated initializers isn't particularly nice,
but I guess it's valid, and the simple alternatives are probably worse.

Thanks,
drew
Dave Martin - April 4, 2019, 8:06 a.m.
On Wed, Apr 03, 2019 at 09:39:43PM +0200, Andrew Jones wrote:
> On Fri, Mar 29, 2019 at 01:00:37PM +0000, Dave Martin wrote:
> > This patch adds the necessary support for context switching ZCR_EL1
> > for each vcpu.
> > 
> > ZCR_EL1 is trapped alongside the FPSIMD/SVE registers, so it makes
> > sense for it to be handled as part of the guest FPSIMD/SVE context
> > for context switch purposes instead of handling it as a general
> > system register.  This means that it can be switched in lazily at
> > the appropriate time.  No effort is made to track host context for
> > this register, since SVE requires VHE: thus the hosts's value for
> > this register lives permanently in ZCR_EL2 and does not alias the
> > guest's value at any time.
> > 
> > The Hyp switch and fpsimd context handling code is extended
> > appropriately.
> > 
> > Accessors are added in sys_regs.c to expose the SVE system
> > registers and ID register fields.  Because these need to be
> > conditionally visible based on the guest configuration, they are
> > implemented separately for now rather than by use of the generic
> > system register helpers.  This may be abstracted better later on
> > when/if there are more features requiring this model.
> > 
> > ID_AA64ZFR0_EL1 is RO-RAZ for MRS/MSR when SVE is disabled for the
> > guest, but for compatibility with non-SVE aware KVM implementations
> > the register should not be enumerated at all for KVM_GET_REG_LIST
> > in this case.  For consistency we also reject ioctl access to the
> > register.  This ensures that a non-SVE-enabled guest looks the same
> > to userspace, irrespective of whether the kernel KVM implementation
> > supports SVE.
> > 
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> > Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> > 
> > ---
> > 
> > Changes since v5:
> > 
> >  * Port to the renamed visibility() framework.
> > 
> >  * Swap visiblity() helpers so that they appear by the relevant accessor
> >    functions.
> > 
> >  * [Julien Grall] With the visibility() checks, {get,set}_zcr_el1()
> >    degenerate to doing exactly what the common code does, so drop them.
> > 
> >    The ID_AA64ZFR0_EL1 handlers are still needed to provide contitional
> >    RAZ behaviour.  This could be moved to the common code too, but since
> >    this is a one-off case I don't do this for now.  We can address this
> >    later if other regs need to follow the same pattern.
> > 
> >  * [Julien Thierry] Reset ZCR_EL1 to a fixed value using reset_val
> >    instead of using relying on reset_unknown() honouring set bits in val
> >    as RES0.
> > 
> >    Most of the bits in ZCR_EL1 are RES0 anyway, and many implementations
> >    of SVE will support larger vectors than 128 bits, so 0 seems as good
> >    a value as any to expose guests that forget to initialise this
> >    register properly.
> > ---

[...]

> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index 3563fe6..9d46066 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c

[...]

> > +static int get_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> > +		const struct sys_reg_desc *rd,
> > +		const struct kvm_one_reg *reg, void __user *uaddr)
> > +{
> > +	u64 val;
> > +
> > +	if (!vcpu_has_sve(vcpu))
> > +		return -ENOENT;
> 
> This shouldn't be necessary. The visibility check in
> kvm_arm_sys_reg_get_reg already covers it.
> 
> > +
> > +	val = guest_id_aa64zfr0_el1(vcpu);
> > +	return reg_to_user(uaddr, &val, reg->id);
> > +}
> > +
> > +static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> > +		const struct sys_reg_desc *rd,
> > +		const struct kvm_one_reg *reg, void __user *uaddr)
> > +{
> > +	const u64 id = sys_reg_to_index(rd);
> > +	int err;
> > +	u64 val;
> > +
> > +	if (!vcpu_has_sve(vcpu))
> > +		return -ENOENT;
> 
> Also not necessary.

Hmm, true.  Because the logic is a bit spread out I felt uneasy with
simply deleting these checks, but if they fire, something has
definitely gone wrong elsewhere.

In its current form the code makes it look like it could be legitimate
to get here with !vcpu_has_sve(vcpu), which is misleading.

What if we demote these to WARN_ON()?  This isn't a fast path.

[...]

> >  /*
> >   * cpufeature ID register user accessors
> >   *
> > @@ -1346,7 +1418,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >  	ID_SANITISED(ID_AA64PFR1_EL1),
> >  	ID_UNALLOCATED(4,2),
> >  	ID_UNALLOCATED(4,3),
> > -	ID_UNALLOCATED(4,4),
> > +	{ SYS_DESC(SYS_ID_AA64ZFR0_EL1), access_id_aa64zfr0_el1, .get_user = get_id_aa64zfr0_el1, .set_user = set_id_aa64zfr0_el1, .visibility = sve_id_visibility },
> >  	ID_UNALLOCATED(4,5),
> >  	ID_UNALLOCATED(4,6),
> >  	ID_UNALLOCATED(4,7),
> > @@ -1383,6 +1455,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >  
> >  	{ SYS_DESC(SYS_SCTLR_EL1), access_vm_reg, reset_val, SCTLR_EL1, 0x00C50078 },
> >  	{ SYS_DESC(SYS_CPACR_EL1), NULL, reset_val, CPACR_EL1, 0 },
> > +	{ SYS_DESC(SYS_ZCR_EL1), NULL, reset_val, ZCR_EL1, 0, .visibility = sve_visibility },
> >  	{ SYS_DESC(SYS_TTBR0_EL1), access_vm_reg, reset_unknown, TTBR0_EL1 },
> >  	{ SYS_DESC(SYS_TTBR1_EL1), access_vm_reg, reset_unknown, TTBR1_EL1 },
> >  	{ SYS_DESC(SYS_TCR_EL1), access_vm_reg, reset_val, TCR_EL1, 0 },
> 
> Mixing designated and non-designated initializers isn't particularly nice,
> but I guess it's valid, and the simple alternatives are probably worse.

Agreed.  We could add designators everywhere or more wrapper macros, but
I couldn't see any option that didn't have drawbacks.

Since this is a rare case today, I preferred to keep it as explicit in
the source as possible.

Cheers
---Dave
Andrew Jones - April 4, 2019, 8:32 a.m.
On Thu, Apr 04, 2019 at 09:06:58AM +0100, Dave Martin wrote:
> On Wed, Apr 03, 2019 at 09:39:43PM +0200, Andrew Jones wrote:
> > On Fri, Mar 29, 2019 at 01:00:37PM +0000, Dave Martin wrote:
> > > This patch adds the necessary support for context switching ZCR_EL1
> > > for each vcpu.
> > > 
> > > ZCR_EL1 is trapped alongside the FPSIMD/SVE registers, so it makes
> > > sense for it to be handled as part of the guest FPSIMD/SVE context
> > > for context switch purposes instead of handling it as a general
> > > system register.  This means that it can be switched in lazily at
> > > the appropriate time.  No effort is made to track host context for
> > > this register, since SVE requires VHE: thus the hosts's value for
> > > this register lives permanently in ZCR_EL2 and does not alias the
> > > guest's value at any time.
> > > 
> > > The Hyp switch and fpsimd context handling code is extended
> > > appropriately.
> > > 
> > > Accessors are added in sys_regs.c to expose the SVE system
> > > registers and ID register fields.  Because these need to be
> > > conditionally visible based on the guest configuration, they are
> > > implemented separately for now rather than by use of the generic
> > > system register helpers.  This may be abstracted better later on
> > > when/if there are more features requiring this model.
> > > 
> > > ID_AA64ZFR0_EL1 is RO-RAZ for MRS/MSR when SVE is disabled for the
> > > guest, but for compatibility with non-SVE aware KVM implementations
> > > the register should not be enumerated at all for KVM_GET_REG_LIST
> > > in this case.  For consistency we also reject ioctl access to the
> > > register.  This ensures that a non-SVE-enabled guest looks the same
> > > to userspace, irrespective of whether the kernel KVM implementation
> > > supports SVE.
> > > 
> > > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > > Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> > > Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> > > 
> > > ---
> > > 
> > > Changes since v5:
> > > 
> > >  * Port to the renamed visibility() framework.
> > > 
> > >  * Swap visiblity() helpers so that they appear by the relevant accessor
> > >    functions.
> > > 
> > >  * [Julien Grall] With the visibility() checks, {get,set}_zcr_el1()
> > >    degenerate to doing exactly what the common code does, so drop them.
> > > 
> > >    The ID_AA64ZFR0_EL1 handlers are still needed to provide contitional
> > >    RAZ behaviour.  This could be moved to the common code too, but since
> > >    this is a one-off case I don't do this for now.  We can address this
> > >    later if other regs need to follow the same pattern.
> > > 
> > >  * [Julien Thierry] Reset ZCR_EL1 to a fixed value using reset_val
> > >    instead of using relying on reset_unknown() honouring set bits in val
> > >    as RES0.
> > > 
> > >    Most of the bits in ZCR_EL1 are RES0 anyway, and many implementations
> > >    of SVE will support larger vectors than 128 bits, so 0 seems as good
> > >    a value as any to expose guests that forget to initialise this
> > >    register properly.
> > > ---
> 
> [...]
> 
> > > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > > index 3563fe6..9d46066 100644
> > > --- a/arch/arm64/kvm/hyp/switch.c
> > > +++ b/arch/arm64/kvm/hyp/switch.c
> 
> [...]
> 
> > > +static int get_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> > > +		const struct sys_reg_desc *rd,
> > > +		const struct kvm_one_reg *reg, void __user *uaddr)
> > > +{
> > > +	u64 val;
> > > +
> > > +	if (!vcpu_has_sve(vcpu))
> > > +		return -ENOENT;
> > 
> > This shouldn't be necessary. The visibility check in
> > kvm_arm_sys_reg_get_reg already covers it.
> > 
> > > +
> > > +	val = guest_id_aa64zfr0_el1(vcpu);
> > > +	return reg_to_user(uaddr, &val, reg->id);
> > > +}
> > > +
> > > +static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
> > > +		const struct sys_reg_desc *rd,
> > > +		const struct kvm_one_reg *reg, void __user *uaddr)
> > > +{
> > > +	const u64 id = sys_reg_to_index(rd);
> > > +	int err;
> > > +	u64 val;
> > > +
> > > +	if (!vcpu_has_sve(vcpu))
> > > +		return -ENOENT;
> > 
> > Also not necessary.
> 
> Hmm, true.  Because the logic is a bit spread out I felt uneasy with
> simply deleting these checks, but if they fire, something has
> definitely gone wrong elsewhere.
> 
> In its current form the code makes it look like it could be legitimate
> to get here with !vcpu_has_sve(vcpu), which is misleading.
> 
> What if we demote these to WARN_ON()?  This isn't a fast path.

A WARN_ON sounds good to me.

Thanks,
drew
Andrew Jones - April 4, 2019, 8:59 a.m.
On Fri, Mar 29, 2019 at 01:00:37PM +0000, Dave Martin wrote:
> This patch adds the necessary support for context switching ZCR_EL1
> for each vcpu.
> 
> ZCR_EL1 is trapped alongside the FPSIMD/SVE registers, so it makes
> sense for it to be handled as part of the guest FPSIMD/SVE context
> for context switch purposes instead of handling it as a general
> system register.  This means that it can be switched in lazily at
> the appropriate time.  No effort is made to track host context for
> this register, since SVE requires VHE: thus the hosts's value for
> this register lives permanently in ZCR_EL2 and does not alias the
> guest's value at any time.
> 
> The Hyp switch and fpsimd context handling code is extended
> appropriately.
> 
> Accessors are added in sys_regs.c to expose the SVE system
> registers and ID register fields.  Because these need to be
> conditionally visible based on the guest configuration, they are
> implemented separately for now rather than by use of the generic
> system register helpers.  This may be abstracted better later on
> when/if there are more features requiring this model.
> 
> ID_AA64ZFR0_EL1 is RO-RAZ for MRS/MSR when SVE is disabled for the
> guest, but for compatibility with non-SVE aware KVM implementations
> the register should not be enumerated at all for KVM_GET_REG_LIST
> in this case.  For consistency we also reject ioctl access to the
> register.  This ensures that a non-SVE-enabled guest looks the same
> to userspace, irrespective of whether the kernel KVM implementation
> supports SVE.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> 
> ---
> 
> Changes since v5:
> 
>  * Port to the renamed visibility() framework.
> 
>  * Swap visiblity() helpers so that they appear by the relevant accessor
>    functions.
> 
>  * [Julien Grall] With the visibility() checks, {get,set}_zcr_el1()
>    degenerate to doing exactly what the common code does, so drop them.
> 
>    The ID_AA64ZFR0_EL1 handlers are still needed to provide contitional
>    RAZ behaviour.  This could be moved to the common code too, but since
>    this is a one-off case I don't do this for now.  We can address this
>    later if other regs need to follow the same pattern.
> 
>  * [Julien Thierry] Reset ZCR_EL1 to a fixed value using reset_val
>    instead of using relying on reset_unknown() honouring set bits in val
>    as RES0.
> 
>    Most of the bits in ZCR_EL1 are RES0 anyway, and many implementations
>    of SVE will support larger vectors than 128 bits, so 0 seems as good
>    a value as any to expose guests that forget to initialise this
>    register properly.
> ---
>  arch/arm64/include/asm/kvm_host.h |  1 +
>  arch/arm64/include/asm/sysreg.h   |  3 ++
>  arch/arm64/kvm/fpsimd.c           |  9 ++++-
>  arch/arm64/kvm/hyp/switch.c       |  3 ++
>  arch/arm64/kvm/sys_regs.c         | 83 ++++++++++++++++++++++++++++++++++++---
>  5 files changed, 93 insertions(+), 6 deletions(-)
>

Reviewed-by: Andrew Jones <drjones@redhat.com>

Patch

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index ad4f7f0..22cf484 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -121,6 +121,7 @@  enum vcpu_sysreg {
 	SCTLR_EL1,	/* System Control Register */
 	ACTLR_EL1,	/* Auxiliary Control Register */
 	CPACR_EL1,	/* Coprocessor Access Control */
+	ZCR_EL1,	/* SVE Control */
 	TTBR0_EL1,	/* Translation Table Base Register 0 */
 	TTBR1_EL1,	/* Translation Table Base Register 1 */
 	TCR_EL1,	/* Translation Control Register */
diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 5b267de..4d6262d 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -454,6 +454,9 @@ 
 #define SYS_ICH_LR14_EL2		__SYS__LR8_EL2(6)
 #define SYS_ICH_LR15_EL2		__SYS__LR8_EL2(7)
 
+/* VHE encodings for architectural EL0/1 system registers */
+#define SYS_ZCR_EL12			sys_reg(3, 5, 1, 2, 0)
+
 /* Common SCTLR_ELx flags. */
 #define SCTLR_ELx_DSSBS	(_BITUL(44))
 #define SCTLR_ELx_ENIA	(_BITUL(31))
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 1cf4f02..7053bf4 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -103,14 +103,21 @@  void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
 void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 {
 	unsigned long flags;
+	bool host_has_sve = system_supports_sve();
+	bool guest_has_sve = vcpu_has_sve(vcpu);
 
 	local_irq_save(flags);
 
 	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
+		u64 *guest_zcr = &vcpu->arch.ctxt.sys_regs[ZCR_EL1];
+
 		/* Clean guest FP state to memory and invalidate cpu view */
 		fpsimd_save();
 		fpsimd_flush_cpu_state();
-	} else if (system_supports_sve()) {
+
+		if (guest_has_sve)
+			*guest_zcr = read_sysreg_s(SYS_ZCR_EL12);
+	} else if (host_has_sve) {
 		/*
 		 * The FPSIMD/SVE state in the CPU has not been touched, and we
 		 * have SVE (and VHE): CPACR_EL1 (alias CPTR_EL2) has been
diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 3563fe6..9d46066 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -351,6 +351,9 @@  static bool __hyp_text __hyp_switch_fpsimd(struct kvm_vcpu *vcpu)
 
 	__fpsimd_restore_state(&vcpu->arch.ctxt.gp_regs.fp_regs);
 
+	if (vcpu_has_sve(vcpu))
+		write_sysreg_s(vcpu->arch.ctxt.sys_regs[ZCR_EL1], SYS_ZCR_EL12);
+
 	/* Skip restoring fpexc32 for AArch64 guests */
 	if (!(read_sysreg(hcr_el2) & HCR_RW))
 		write_sysreg(vcpu->arch.ctxt.sys_regs[FPEXC32_EL2],
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c86a7b0..09e9b06 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1051,10 +1051,7 @@  static u64 read_id_reg(const struct kvm_vcpu *vcpu,
 			 (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
 	u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
 
-	if (id == SYS_ID_AA64PFR0_EL1) {
-		if (val & (0xfUL << ID_AA64PFR0_SVE_SHIFT))
-			kvm_debug("SVE unsupported for guests, suppressing\n");
-
+	if (id == SYS_ID_AA64PFR0_EL1 && !vcpu_has_sve(vcpu)) {
 		val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
 	} else if (id == SYS_ID_AA64ISAR1_EL1) {
 		const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
@@ -1101,6 +1098,81 @@  static int reg_from_user(u64 *val, const void __user *uaddr, u64 id);
 static int reg_to_user(void __user *uaddr, const u64 *val, u64 id);
 static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
 
+/* Visibility overrides for SVE-specific control registers */
+static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
+				   const struct sys_reg_desc *rd)
+{
+	if (vcpu_has_sve(vcpu))
+		return 0;
+
+	return REG_HIDDEN_USER | REG_HIDDEN_GUEST;
+}
+
+/* Visibility overrides for SVE-specific ID registers */
+static unsigned int sve_id_visibility(const struct kvm_vcpu *vcpu,
+				      const struct sys_reg_desc *rd)
+{
+	if (vcpu_has_sve(vcpu))
+		return 0;
+
+	return REG_HIDDEN_USER;
+}
+
+/* Generate the emulated ID_AA64ZFR0_EL1 value exposed to the guest */
+static u64 guest_id_aa64zfr0_el1(const struct kvm_vcpu *vcpu)
+{
+	if (!vcpu_has_sve(vcpu))
+		return 0;
+
+	return read_sanitised_ftr_reg(SYS_ID_AA64ZFR0_EL1);
+}
+
+static bool access_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
+				   struct sys_reg_params *p,
+				   const struct sys_reg_desc *rd)
+{
+	if (p->is_write)
+		return write_to_read_only(vcpu, p, rd);
+
+	p->regval = guest_id_aa64zfr0_el1(vcpu);
+	return true;
+}
+
+static int get_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
+		const struct sys_reg_desc *rd,
+		const struct kvm_one_reg *reg, void __user *uaddr)
+{
+	u64 val;
+
+	if (!vcpu_has_sve(vcpu))
+		return -ENOENT;
+
+	val = guest_id_aa64zfr0_el1(vcpu);
+	return reg_to_user(uaddr, &val, reg->id);
+}
+
+static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
+		const struct sys_reg_desc *rd,
+		const struct kvm_one_reg *reg, void __user *uaddr)
+{
+	const u64 id = sys_reg_to_index(rd);
+	int err;
+	u64 val;
+
+	if (!vcpu_has_sve(vcpu))
+		return -ENOENT;
+
+	err = reg_from_user(&val, uaddr, id);
+	if (err)
+		return err;
+
+	/* This is what we mean by invariant: you can't change it. */
+	if (val != guest_id_aa64zfr0_el1(vcpu))
+		return -EINVAL;
+
+	return 0;
+}
+
 /*
  * cpufeature ID register user accessors
  *
@@ -1346,7 +1418,7 @@  static const struct sys_reg_desc sys_reg_descs[] = {
 	ID_SANITISED(ID_AA64PFR1_EL1),
 	ID_UNALLOCATED(4,2),
 	ID_UNALLOCATED(4,3),
-	ID_UNALLOCATED(4,4),
+	{ SYS_DESC(SYS_ID_AA64ZFR0_EL1), access_id_aa64zfr0_el1, .get_user = get_id_aa64zfr0_el1, .set_user = set_id_aa64zfr0_el1, .visibility = sve_id_visibility },
 	ID_UNALLOCATED(4,5),
 	ID_UNALLOCATED(4,6),
 	ID_UNALLOCATED(4,7),
@@ -1383,6 +1455,7 @@  static const struct sys_reg_desc sys_reg_descs[] = {
 
 	{ SYS_DESC(SYS_SCTLR_EL1), access_vm_reg, reset_val, SCTLR_EL1, 0x00C50078 },
 	{ SYS_DESC(SYS_CPACR_EL1), NULL, reset_val, CPACR_EL1, 0 },
+	{ SYS_DESC(SYS_ZCR_EL1), NULL, reset_val, ZCR_EL1, 0, .visibility = sve_visibility },
 	{ SYS_DESC(SYS_TTBR0_EL1), access_vm_reg, reset_unknown, TTBR0_EL1 },
 	{ SYS_DESC(SYS_TTBR1_EL1), access_vm_reg, reset_unknown, TTBR1_EL1 },
 	{ SYS_DESC(SYS_TCR_EL1), access_vm_reg, reset_val, TCR_EL1, 0 },