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

Xiaoyao Li
2025-02-20T10:50:26Z

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.

> /*
> * 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: