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

[Yang, Weijiang]

On 10/8/2023 2:19 PM, Chao Gao wrote:
> 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?

As you can see MSR_KVM_SSP is not purely emulated MSR, it's linked to VMCS field(GUEST_SSP),
IMO, the check is necessary, in other words, no need to expose it when SHSTK is not supported
by KVM.