From 898a811f1486df3209ce857027d0564d6a616cf2 Mon Sep 17 00:00:00 2001
From: Jim Mattson <jmattson@google.com>
Date: Wed, 5 Dec 2018 15:28:59 -0800
Subject: [PATCH] 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>
---
arch/x86/kvm/vmx/vmx.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c1feb62faac4c..d754fd0c86743 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
--
2.30.2