On 10/9/2024 6:45 PM, Tom Lendacky wrote: > On 10/9/24 01:01, Neeraj Upadhyay wrote: >> >> >> On 10/9/2024 10:53 AM, Borislav Petkov wrote: >>> On Wed, Oct 09, 2024 at 07:26:55AM +0530, Neeraj Upadhyay wrote: >>>> As SECURE_AVIC feature is not supported (as reported by snp_get_unsupported_features()) >>>> by guest at this patch in the series, it is added to SNP_FEATURES_IMPL_REQ here. The bit >>>> value within SNP_FEATURES_IMPL_REQ hasn't changed with this change as the same bit pos >>>> was part of MSR_AMD64_SNP_RESERVED_MASK before this patch. In patch 14 SECURE_AVIC guest >>>> support is indicated by guest. >>> >>> So what's the point of adding it to SNP_FEATURES_IMPL_REQ here? What does that >>> do at all in this patch alone? Why is this change needed in here? >>> >> >> Before this patch, if hypervisor enables Secure AVIC (reported in sev_status), guest would >> terminate in snp_check_features(). The reason for this is, SNP_FEATURES_IMPL_REQ had the Secure >> AVIC bit set before this patch, as that bit was part of MSR_AMD64_SNP_RESERVED_MASK >> GENMASK_ULL(63, 18). >> >> #define SNP_FEATURES_IMPL_REQ (MSR_AMD64_SNP_VTOM | \ >> ... >> MSR_AMD64_SNP_RESERVED_MASK) >> >> >> >> Adding MSR_AMD64_SNP_SECURE_AVIC_BIT (bit 18) to SNP_FEATURES_IMPL_REQ in this patch >> keeps that behavior intact as now with this change MSR_AMD64_SNP_RESERVED_MASK becomes >> GENMASK_ULL(63, 19). >> >> >>> IOW, why don't you do all the feature bit handling in the last patch, where it >>> all belongs logically? >>> >> >> If we do that, then hypervisor could have enabled Secure AVIC support and the guest >> code at this patch won't catch the missing guest-support early and it can result in some >> unknown failures at later point during guest boot. > > Won't the SNP_RESERVED_MASK catch it? You are just renaming the bit > position value, right? It was a 1 before and is still a 1. So the guest > will terminate if the hypervisor sets the Secure AVIC bit both before > and after this patch, right? > Yes that is right. SNP_RESERVED_MASK catches it before this patch. My reply to Boris above was for the case if we move setting of MSR_AMD64_SNP_SECURE_AVIC_ENABLED in SNP_FEATURES_IMPL_REQ from this patch to patch 14. - Neeraj > Thanks, > Tom > >> >> >> - Neeraj >> >>> In the last patch you can start *testing* for >>> MSR_AMD64_SNP_SECURE_AVIC_ENABLED *and* enforce it with SNP_FEATURES_PRESENT. >>>
