Re: [PATCH v2 03/15] KVM: x86: Refresh CPUID on writes to MSR_IA32_XSS

Yang, Weijiang
2023-01-30T06:33:53Z

On 1/27/2023 3:50 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
>> Updated CPUID.0xD.0x1, which reports the current required storage size
>> of all features enabled via XCR0 | XSS, when the guest's XSS is modified.
>>
>> Note, KVM does not yet support any XSS based features, i.e. supported_xss
>> is guaranteed to be zero at this time.
>>
>> Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
>> Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> ---
>> arch/x86/kvm/cpuid.c | 16 +++++++++++++---
>> arch/x86/kvm/x86.c | 6 ++++--
>> 2 files changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 6b5912578edd..85e3df6217af 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -272,9 +272,19 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>> best->ebx = xstate_required_size(vcpu->arch.xcr0, false);
>>
>> best = cpuid_entry2_find(entries, nent, 0xD, 1);
>> - if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>> - cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
>> - best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
>> + if (best) {
>> + if (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>> + cpuid_entry_has(best, X86_FEATURE_XSAVEC)) {
>> + u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
>> +
>> + best->ebx = xstate_required_size(xstate, true);
>> + }
>> +
>> + if (!cpuid_entry_has(best, X86_FEATURE_XSAVES)) {
>> + best->ecx = 0;
>> + best->edx = 0;
> ECX and EDX should be left alone, it is userspace's responsibility to provide a
> sane CPUID model. E.g. KVM doesn't clear EBX or EDX in CPUID.0xD.0x1 when XSAVE
> is unsupported.

Thanks for review!

Sure, I'll remove it.

>
>> + }
>> + }
>>
>> best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
>> if (kvm_hlt_in_guest(vcpu->kvm) && best &&
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 16726b44061b..888a153e32bc 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3685,8 +3685,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> */
>> if (data & ~kvm_caps.supported_xss)
>> return 1;
>> - vcpu->arch.ia32_xss = data;
>> - kvm_update_cpuid_runtime(vcpu);
>> + if (vcpu->arch.ia32_xss != data) {
>> + vcpu->arch.ia32_xss = data;
>> + kvm_update_cpuid_runtime(vcpu);
>> + }
>> break;
>> case MSR_SMI_COUNT:
>> if (!msr_info->host_initiated)
>> --
>> 2.27.0
>>