Re: [PATCH v16 09/51] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs

[Binbin Wu]

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
[...]
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3e66d8c5000a..ae402463f991 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -136,6 +136,9 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>   static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2);
>   
>   static DEFINE_MUTEX(vendor_module_lock);
> +static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> +static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
> +
>   struct kvm_x86_ops kvm_x86_ops __read_mostly;
>   
>   #define KVM_X86_OP(func)					     \
> @@ -3801,6 +3804,67 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>   	mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa));
>   }
>   
> +/*
> + * Returns true if the MSR in question is managed via XSTATE, i.e. is context
> + * switched with the rest of guest FPU state.  Note!  S_CET is _not_ context
> + * switched via XSTATE even though it _is_ saved/restored via XSAVES/XRSTORS.
> + * Because S_CET is loaded on VM-Enter and VM-Exit via dedicated VMCS fields,
> + * the value saved/restored via XSTATE is always the host's value.  That detail
> + * is _extremely_ important, as the guest's S_CET must _never_ be resident in
> + * hardware while executing in the host.  Loading guest values for U_CET and
> + * PL[0-3]_SSP while executing in the kernel is safe, as U_CET is specific to
> + * userspace, and PL[0-3]_SSP are only consumed when transitioning to lower
> + * privilegel levels, i.e. are effectively only consumed by userspace as well.

s/privilegel/privilege[...]