Re: [PATCH v15 29/41] KVM: SEV: Synchronize MSR_IA32_XSS from the GHCB when it's valid

[John Allen]

On Fri, Sep 12, 2025 at 04:23:07PM -0700, Sean Christopherson wrote:
> Synchronize XSS from the GHCB to KVM's internal tracking if the guest
> marks XSS as valid on a #VMGEXIT.  Like XCR0, KVM needs an up-to-date copy
> of XSS in order to compute the required XSTATE size when emulating
> CPUID.0xD.0x1 for the guest.
> 
> Treat the incoming XSS change as an emulated write, i.e. validatate the
> guest-provided value, to avoid letting the guest load garbage into KVM's
> tracking.  Simply ignore bad values, as either the guest managed to get an
> unsupported value into hardware, or the guest is misbehaving and providing
> pure garbage.  In either case, KVM can't fix the broken guest.
> 
> Note, emulating the change as an MSR write also takes care of side effects,
> e.g. marking dynamic CPUID bits as dirty.
> 
> Suggested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/sev.c | 3 +++
>  arch/x86/kvm/svm/svm.h | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 0cd77a87dd84..0cd32df7b9b6 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3306,6 +3306,9 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
>  	if (kvm_ghcb_xcr0_is_valid(svm))
>  		__kvm_set_xcr(vcpu, 0, kvm_ghcb_get_xcr0(ghcb));
>  
> +	if (kvm_ghcb_xss_is_valid(svm))
> +		__kvm_emulate_msr_write(vcpu, MSR_IA32_XSS, kvm_ghcb_get_xss(ghcb));
> +

It looks like this is the change that caused the selftest regression
with sev-es. It's not yet clear to me what the problem is though.

Thanks,
John