Patchwork [17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD

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Submitter Sebastian Andrzej Siewior
Date Jan. 9, 2019, 11:47 a.m.
Message ID <20190109114744.10936-18-bigeasy@linutronix.de>
Download mbox | patch
Permalink /patch/695739/
State New
Headers show

Comments

Sebastian Andrzej Siewior - Jan. 9, 2019, 11:47 a.m.
From: Rik van Riel <riel@surriel.com>

The FPU registers need only to be saved if TIF_NEED_FPU_LOAD is not set.
Otherwise this has been already done and can be skipped.

Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/x86/kernel/fpu/signal.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)
Borislav Petkov - Jan. 30, 2019, 11:56 a.m.
On Wed, Jan 09, 2019 at 12:47:39PM +0100, Sebastian Andrzej Siewior wrote:
> From: Rik van Riel <riel@surriel.com>
> 
> The FPU registers need only to be saved if TIF_NEED_FPU_LOAD is not set.
> Otherwise this has been already done and can be skipped.
> 
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  arch/x86/kernel/fpu/signal.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index bf4e6caad305e..a25be217f9a2c 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -156,7 +156,16 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
>  			sizeof(struct user_i387_ia32_struct), NULL,
>  			(struct _fpstate_32 __user *) buf) ? -1 : 1;
>  
> -	copy_fpregs_to_fpstate(fpu);
> +	__fpregs_changes_begin();
> +	/*
> +	 * If we do not need to load the FPU registers at return to userspace
> +	 * then the CPU has the current state and we need to save it. Otherwise
> +	 * it is already done and we can skip it.
> +	 */
> +	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
> +		copy_fpregs_to_fpstate(fpu);

I wonder if this flag would make the code more easy to follow by calling
it

	TIF_FPU_REGS_VALID

instead, to denote that the FPU registers in the CPU have a valid
content.

Then the test becomes:

	if (test_thread_flag(TIF_FPU_REGS_VALID))
		copy_fpregs_to_fpstate(fpu);
Sebastian Andrzej Siewior - Jan. 30, 2019, 12:28 p.m.
On 2019-01-30 12:56:14 [+0100], Borislav Petkov wrote:
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index bf4e6caad305e..a25be217f9a2c 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -156,7 +156,16 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
> >  			sizeof(struct user_i387_ia32_struct), NULL,
> >  			(struct _fpstate_32 __user *) buf) ? -1 : 1;
> >  
> > -	copy_fpregs_to_fpstate(fpu);
> > +	__fpregs_changes_begin();
> > +	/*
> > +	 * If we do not need to load the FPU registers at return to userspace
> > +	 * then the CPU has the current state and we need to save it. Otherwise
> > +	 * it is already done and we can skip it.
> > +	 */
> > +	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
> > +		copy_fpregs_to_fpstate(fpu);
> 
> I wonder if this flag would make the code more easy to follow by calling
> it
> 
> 	TIF_FPU_REGS_VALID
> 
> instead, to denote that the FPU registers in the CPU have a valid
> content.
> 
> Then the test becomes:
> 
> 	if (test_thread_flag(TIF_FPU_REGS_VALID))
> 		copy_fpregs_to_fpstate(fpu);

I've been asked to add comment above the sequence so it is understood. I
think the general approach is easy to follow once the concept is
understood. I don't mind renaming the TIF_ thingy once again (it
happend once or twice and I think the current one was suggested by Andy
unless I mixed things up).
The problem I have with the above is that

	if (test_thread_flag(TIF_NEED_FPU_LOAD))
		do_that()

becomes
	if (!test_thread_flag(TIF_FPU_REGS_VALID))
		do_that()

and you could argue again the other way around. So do we want NEED_LOAD
or NEED_SAVE flag which is another way of saying REGS_VALID?
More importantly the logic is changed when the bit is set and this
requires more thinking than just doing sed on the patch series.

Sebastian
Borislav Petkov - Jan. 30, 2019, 12:53 p.m.
On Wed, Jan 30, 2019 at 01:28:20PM +0100, Sebastian Andrzej Siewior wrote:
> > > +	/*
> > > +	 * If we do not need to load the FPU registers at return to userspace
> > > +	 * then the CPU has the current state and we need to save it. Otherwise
> > > +	 * it is already done and we can skip it.
> > > +	 */
> > > +	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
> > > +		copy_fpregs_to_fpstate(fpu);
> > 
> > I wonder if this flag would make the code more easy to follow by calling
> > it
> > 
> > 	TIF_FPU_REGS_VALID
> > 
> > instead, to denote that the FPU registers in the CPU have a valid
> > content.
> > 
> > Then the test becomes:
> > 
> > 	if (test_thread_flag(TIF_FPU_REGS_VALID))
> > 		copy_fpregs_to_fpstate(fpu);
> 
> I've been asked to add comment above the sequence so it is understood. I
> think the general approach is easy to follow once the concept is
> understood. I don't mind renaming the TIF_ thingy once again (it
> happend once or twice and I think the current one was suggested by Andy
> unless I mixed things up).
> The problem I have with the above is that
> 
> 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> 		do_that()
> 
> becomes
> 	if (!test_thread_flag(TIF_FPU_REGS_VALID))
> 		do_that()

Err, above it becomes

	if (test_thread_flag(TIF_FPU_REGS_VALID))
		copy_fpregs_to_fpstate(fpu);

without the "!". I.e., CPU's FPU regs are valid and we need to save them.

Or am I misreading the comment above?

> and you could argue again the other way around. So do we want NEED_LOAD
> or NEED_SAVE flag which is another way of saying REGS_VALID?

All fine and dandy except NEED_FPU_LOAD is ambiguous to me: we need to
load them where? Into the CPU? Or into the FPU state save area?

TIF_FPU_REGS_VALID is clearer in the sense that the CPU's FPU registers
are currently valid for the current task. As there are no other FPU
registers except the CPU's.

> More importantly the logic is changed when the bit is set and this
> requires more thinking than just doing sed on the patch series.

Sure.

And I'll get accustomed to the logic whatever the name is - this is just
a "wouldn't it be better" thing.

Thx.
Sebastian Andrzej Siewior - Feb. 7, 2019, 2:10 p.m.
On 2019-01-30 13:53:51 [+0100], Borislav Petkov wrote:
> > I've been asked to add comment above the sequence so it is understood. I
> > think the general approach is easy to follow once the concept is
> > understood. I don't mind renaming the TIF_ thingy once again (it
> > happend once or twice and I think the current one was suggested by Andy
> > unless I mixed things up).
> > The problem I have with the above is that
> > 
> > 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > 		do_that()
> > 
> > becomes
> > 	if (!test_thread_flag(TIF_FPU_REGS_VALID))
> > 		do_that()
> 
> Err, above it becomes
> 
> 	if (test_thread_flag(TIF_FPU_REGS_VALID))
> 		copy_fpregs_to_fpstate(fpu);

The (your) above example yes. But the reverse state
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		do_that()

becomes
 	if (!test_thread_flag(TIF_FPU_REGS_VALID))
 		do_that()

> without the "!". I.e., CPU's FPU regs are valid and we need to save them.
> 
> Or am I misreading the comment above?

Your example is correct. But in the opposite case, when ! was not there
then we have to add it.

> > and you could argue again the other way around. So do we want NEED_LOAD
> > or NEED_SAVE flag which is another way of saying REGS_VALID?
> 
> All fine and dandy except NEED_FPU_LOAD is ambiguous to me: we need to
> load them where? Into the CPU? Or into the FPU state save area?

if you need to LOAD then task-saved-area into CPU-state. If you need to
save it then CPU-state into task-saved-area.

> TIF_FPU_REGS_VALID is clearer in the sense that the CPU's FPU registers
> are currently valid for the current task. As there are no other FPU
> registers except the CPU's.

hmmm. I think it is just taste / habit.

> > More importantly the logic is changed when the bit is set and this
> > requires more thinking than just doing sed on the patch series.
> 
> Sure.
> 
> And I'll get accustomed to the logic whatever the name is - this is just
> a "wouldn't it be better" thing.

If it would be just a name thing then I probably wouldn't mind. But
swapping the logic might break things so I try to avoid that.

> Thx.
> 

Sebastian

Patch

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index bf4e6caad305e..a25be217f9a2c 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -156,7 +156,16 @@  int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 			sizeof(struct user_i387_ia32_struct), NULL,
 			(struct _fpstate_32 __user *) buf) ? -1 : 1;
 
-	copy_fpregs_to_fpstate(fpu);
+	__fpregs_changes_begin();
+	/*
+	 * If we do not need to load the FPU registers at return to userspace
+	 * then the CPU has the current state and we need to save it. Otherwise
+	 * it is already done and we can skip it.
+	 */
+	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
+		copy_fpregs_to_fpstate(fpu);
+
+	__fpregs_changes_end();
 
 	if (using_compacted_format()) {
 		copy_xstate_to_user(buf_fx, xsave, 0, size);