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

["Yang, Weijiang" <weijiang.yang@intel.com>]

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@intel.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> ---
>> 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.