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? 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. >>
