Patchwork [v4,2/5] kvm: vmx: Document the need for MSR_STAR in i386 builds

login
register
mail settings
Submitter Jim Mattson
Date Dec. 5, 2018, 11:28 p.m.
Message ID <20181205232902.1431-3-jmattson@google.com>
Download mbox | patch
Permalink /patch/673593/
State New
Headers show

Comments

Jim Mattson - Dec. 5, 2018, 11:28 p.m.
Add a comment explaining why MSR_STAR must be included in
vmx_msr_index[] even for i386 builds.

The elided comment has not been relevant since move_msr_up() was
introduced in commit a75beee6e4f5d ("KVM: VMX: Avoid saving and
restoring msrs on lightweight vmexit").

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
---
 arch/x86/kvm/vmx.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Patch

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index ad1753e8a85e5..b58c3952c5dfb 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1390,8 +1390,11 @@  static u64 host_efer;
 static void ept_save_pdptrs(struct kvm_vcpu *vcpu);
 
 /*
- * Keep MSR_STAR at the end, as setup_msrs() will try to optimize it
- * away by decrementing the array size.
+ * Though SYSCALL is only supported in 64-bit mode on Intel CPUs, kvm
+ * will emulate SYSCALL in legacy mode if the vendor string in guest
+ * CPUID.0:{EBX,ECX,EDX} is "AuthenticAMD" or "AMDisbetter!" To
+ * support this emulation, IA32_STAR must always be included in
+ * vmx_msr_index[], even in i386 builds.
  */
 static const u32 vmx_msr_index[] = {
 #ifdef CONFIG_X86_64