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

Adrian Hunter
2025-02-27T14:15:19Z

On 25/02/25 07:56, Xiaoyao Li wrote:
> 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.

This patch set is owned by Paolo now, so it is up to him.

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