Re: [PATCH V2 02/12] KVM: x86: Allow the use of kvm_load_host_xsave_state() with guest_state_protected

Paolo Bonzini
2025-03-06T18:04:35Z

On 2/20/25 11:50, Xiaoyao Li wrote:
> On 1/29/2025 5:58 PM, Adrian Hunter wrote:
>> From: Sean Christopherson <seanjc@google.com>
>>
>> Allow the use of kvm_load_host_xsave_state() with
>> vcpu->arch.guest_state_protected == true. This will allow TDX to reuse
>> kvm_load_host_xsave_state() instead of creating its own version.
>>
>> For consistency, amend kvm_load_guest_xsave_state() also.
>>
>> Ensure that guest state that kvm_load_host_xsave_state() depends upon,
>> such as MSR_IA32_XSS, cannot be changed by user space, if
>> guest_state_protected.
>>
>> [Adrian: wrote commit message]
>>
>> Link: https://lore.kernel.org/r/Z2GiQS_RmYeHU09L@google.com
>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
>> ---
>> TD vcpu enter/exit v2:
>>   - New patch
>> ---
>>   arch/x86/kvm/svm/svm.c |  7 +++++--
>>   arch/x86/kvm/x86.c     | 18 +++++++++++-------
>>   2 files changed, 16 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 7640a84e554a..b4bcfe15ad5e 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -4253,7 +4253,9 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct
>> kvm_vcpu *vcpu,
>>           svm_set_dr6(svm, DR6_ACTIVE_LOW);
>>       clgi();
>> -    kvm_load_guest_xsave_state(vcpu);
>> +
>> +    if (!vcpu->arch.guest_state_protected)
>> +        kvm_load_guest_xsave_state(vcpu);
>>       kvm_wait_lapic_expire(vcpu);
>> @@ -4282,7 +4284,8 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct
>> kvm_vcpu *vcpu,
>>       if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
>>           kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
>> -    kvm_load_host_xsave_state(vcpu);
>> +    if (!vcpu->arch.guest_state_protected)
>> +        kvm_load_host_xsave_state(vcpu);
>>       stgi();
>>       /* Any pending NMI will happen here */
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index bbb6b7f40b3a..5cf9f023fd4b 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1169,11 +1169,9 @@ EXPORT_SYMBOL_GPL(kvm_lmsw);
>>   void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu)
>>   {
>> -    if (vcpu->arch.guest_state_protected)
>> -        return;
>> +    WARN_ON_ONCE(vcpu->arch.guest_state_protected);
>>       if (kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE)) {
>> -
>>           if (vcpu->arch.xcr0 != kvm_host.xcr0)
>>               xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
>> @@ -1192,13 +1190,11 @@ EXPORT_SYMBOL_GPL(kvm_load_guest_xsave_state);
>>   void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu)
>>   {
>> -    if (vcpu->arch.guest_state_protected)
>> -        return;
>> -
>>       if (cpu_feature_enabled(X86_FEATURE_PKU) &&
>>           ((vcpu->arch.xcr0 & XFEATURE_MASK_PKRU) ||
>>            kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE))) {
>> -        vcpu->arch.pkru = rdpkru();
>> +        if (!vcpu->arch.guest_state_protected)
>> +            vcpu->arch.pkru = rdpkru();
>
> this needs justification.
>
>>           if (vcpu->arch.pkru != vcpu->arch.host_pkru)
>>               wrpkru(vcpu->arch.host_pkru);
>>       }
>
>
>> @@ -3916,6 +3912,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu,
>> struct msr_data *msr_info)
>>           if (!msr_info->host_initiated &&
>>               !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>>               return 1;
>> +
>> +        if (vcpu->arch.guest_state_protected)
>> +            return 1;
>> +
>
> this and below change need to be a separate patch. So that we can
> discuss independently.
>
> I see no reason to make MSR_IA32_XSS special than other MSRs. When
> guest_state_protected, most of the MSRs that aren't emulated by KVM are
> inaccessible by KVM.

I agree with Xiaoyao that this change is sensible but should be proposed
separately for both SNP and TDX.

Allowing the use of kvm_load_host_xsave_state() is really ugly,
especially since the corresponding code is so simple:

if (cpu_feature_enabled(X86_FEATURE_PKU) && vcpu->arch.pkru != 0)
wrpkru(vcpu->arch.host_pkru);

if (kvm_host.xcr0 != kvm_tdx->xfam & kvm_caps.supported_xcr0)
xsetbv(XCR_XFEATURE_ENABLED_MASK, kvm_host.xcr0);

/*
* All TDX hosts support XSS; but even if they didn't, both
* arms of the comparison would be 0 and the wrmsrl would be
* skipped.
*/
if (kvm_host.xss != kvm_tdx->xfam & kvm_caps.supported_xss)
wrmsrl(MSR_IA32_XSS, kvm_host.xss);

This is really all that should be in patch 7. I'll test it and decide
what to do.

Paolo

>>           /*
>>            * KVM supports exposing PT to the guest, but does not support
>>            * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>> @@ -4375,6 +4375,10 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu,
>> struct msr_data *msr_info)
>>           if (!msr_info->host_initiated &&
>>               !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>>               return 1;
>> +
>> +        if (vcpu->arch.guest_state_protected)
>> +            return 1;
>> +
>>           msr_info->data = vcpu->arch.ia32_xss;
>>           break;
>>       case MSR_K7_CLK_CTL:
>
>