Re: [PATCH v14 03/22] KVM: x86: Check XSS validity against guest CPUIDs

[Chao Gao <chao.gao@intel.com>]

On Wed, Sep 10, 2025 at 05:22:15PM +0800, Xiaoyao Li wrote:
>On 9/9/2025 5:39 PM, Chao Gao wrote:
>> Maintain per-guest valid XSS bits and check XSS validity against them
>> rather than against KVM capabilities. This is to prevent bits that are
>> supported by KVM but not supported for a guest from being set.
>> 
>> Opportunistically return KVM_MSR_RET_UNSUPPORTED on IA32_XSS MSR accesses
>> if guest CPUID doesn't enumerate X86_FEATURE_XSAVES. Since
>> KVM_MSR_RET_UNSUPPORTED takes care of host_initiated cases, drop the
>> host_initiated check.
>> 
>> Signed-off-by: Chao Gao <chao.gao@intel.com>
>
>Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
>
><snip>
>> @@ -4011,15 +4011,14 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   		}
>>   		break;
>>   	case MSR_IA32_XSS:
>> -		if (!msr_info->host_initiated &&
>> -		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>> -			return 1;
>> +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>> +			return KVM_MSR_RET_UNSUPPORTED;
>>   		/*
>>   		 * KVM supports exposing PT to the guest, but does not support
>>   		 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>>   		 * XSAVES/XRSTORS to save/restore PT MSRs.
>>   		 */
>
>Not an issue of this patch, there seems not the proper place to put above
>comment.

Agreed.

I am curious why PT state isn't supported, which is apparently missing from
the comment. If it is due to lack of host FPU support, I think the recent
guest-only xfeatures we built for CET can help.

Anyway, PT is only visible on BROKEN kernels. so we won't do anything for
now besides documenting the reason.