From: Jim Mattson <jmattson@google.com>
Date: Wed, 5 Dec 2018 23:28:59 +0000 (-0800)
Subject: kvm: vmx: Document the need for MSR_STAR in i386 builds
X-Git-Tag: v5.4-rc1~1984^2~85
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=898a811f1486df3209ce857027d0564d6a616cf2;p=platform%2Fkernel%2Flinux-rpi.git

kvm: vmx: Document the need for MSR_STAR in i386 builds

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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c1feb62..d754fd0 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -373,8 +373,11 @@ static const struct kvm_vmx_segment_field {
 u64 host_efer;
 
 /*
- * 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.
  */
 const u32 vmx_msr_index[] = {
 #ifdef CONFIG_X86_64