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

Xiaoyao Li
2025-02-25T05:56:50Z

On 2/24/2025 7:38 PM, Adrian Hunter wrote:
> On 20/02/25 12:50, 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.
>
> It was proposed by Sean here:
>
> https://lore.kernel.org/all/Z2WZ091z8GmGjSbC@google.com/
>
> which is part of the email thread referenced by the "Link:" tag above

IMHO, this change needs to be put in patch 07, which is the better place
to justify it.

>>
>>>           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.
>
> Yes, TDX will block access to MSR_IA32_XSS anyway because
> tdx_has_emulated_msr() will return false for MSR_IA32_XSS.
>
> However kvm_load_host_xsave_state() is not TDX-specific code and it
> relies upon vcpu->arch.ia32_xss, so there is reason to block
> access to it when vcpu->arch.guest_state_protected is true.

It is TDX specific logic that TDX requires vcpu->arch.ia32_xss unchanged
since TDX is going to utilize kvm_load_host_xsave_state() to restore
host xsave state and relies on vcpu->arch.ia32_xss to be always the
value of XFAM & XSS_MASK.

So please put this change into the TDX specific patch with the clear
justfication.

>>
>>>           /*
>>>            * 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:
>>
>