Re: [PATCH v6 16/25] KVM: x86: Report KVM supported CET MSRs as to-be-saved

[Chao Gao]

On Thu, Sep 14, 2023 at 02:33:16AM -0400, Yang Weijiang wrote:
>Add CET MSRs to the list of MSRs reported to userspace if the feature,
>i.e. IBT or SHSTK, associated with the MSRs is supported by KVM.
>
>SSP can only be read via RDSSP. Writing even requires destructive and
>potentially faulting operations such as SAVEPREVSSP/RSTORSSP or
>SETSSBSY/CLRSSBSY. Let the host use a pseudo-MSR that is just a wrapper
>for the GUEST_SSP field of the VMCS.
>
>Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
>Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>---
> arch/x86/include/uapi/asm/kvm_para.h |  1 +
> arch/x86/kvm/vmx/vmx.c               |  2 ++
> arch/x86/kvm/x86.c                   | 18 ++++++++++++++++++
> 3 files changed, 21 insertions(+)
>
>diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>index 6e64b27b2c1e..9864bbcf2470 100644
>--- a/arch/x86/include/uapi/asm/kvm_para.h
>+++ b/arch/x86/include/uapi/asm/kvm_para.h
>@@ -58,6 +58,7 @@
> #define MSR_KVM_ASYNC_PF_INT	0x4b564d06
> #define MSR_KVM_ASYNC_PF_ACK	0x4b564d07
> #define MSR_KVM_MIGRATION_CONTROL	0x4b564d08
>+#define MSR_KVM_SSP	0x4b564d09
> 
> struct kvm_steal_time {
> 	__u64 steal;
>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index 72e3943f3693..9409753f45b0 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -7009,6 +7009,8 @@ static bool vmx_has_emulated_msr(struct kvm *kvm, u32 index)
> 	case MSR_AMD64_TSC_RATIO:
> 		/* This is AMD only.  */
> 		return false;
>+	case MSR_KVM_SSP:
>+		return kvm_cpu_cap_has(X86_FEATURE_SHSTK);

For other MSRs in emulated_msrs_all[], KVM doesn't check the associated
CPUID feature bits. Why bother doing this for MSR_KVM_SSP?