On Wed, Mar 30, 2022 at 10:48 PM Nikunj A. Dadhania <nikunj@xxxxxxx> wrote: > > > > On 3/31/2022 1:17 AM, Sean Christopherson wrote: > > On Wed, Mar 30, 2022, Nikunj A. Dadhania wrote: > >> On 3/29/2022 2:30 AM, Sean Christopherson wrote: > >>> Let me preface this by saying I generally like the idea and especially the > >>> performance, but... > >>> > >>> I think we should abandon this approach in favor of committing all our resources > >>> to fd-based private memory[*], which (if done right) will provide on-demand pinning > >>> for "free". > >> > >> I will give this a try for SEV, was on my todo list. > >> > >>> I would much rather get that support merged sooner than later, and use > >>> it as a carrot for legacy SEV to get users to move over to its new APIs, with a long > >>> term goal of deprecating and disallowing SEV/SEV-ES guests without fd-based private > >>> memory. > >> > >>> That would require guest kernel support to communicate private vs. shared, > >> > >> Could you explain this in more detail? This is required for punching hole for shared pages? > > > > Unlike SEV-SNP, which enumerates private vs. shared in the error code, SEV and SEV-ES > > don't provide private vs. shared information to the host (KVM) on page fault. And > > it's even more fundamental then that, as SEV/SEV-ES won't even fault if the guest > > accesses the "wrong" GPA variant, they'll silent consume/corrupt data. > > > > That means KVM can't support implicit conversions for SEV/SEV-ES, and so an explicit > > hypercall is mandatory. SEV doesn't even have a vendor-agnostic guest/host paravirt > > ABI, and IIRC SEV-ES doesn't provide a conversion/map hypercall in the GHCB spec, so > > running a SEV/SEV-ES guest under UPM would require the guest firmware+kernel to be > > properly enlightened beyond what is required architecturally. > > > > So with guest supporting KVM_FEATURE_HC_MAP_GPA_RANGE and host (KVM) supporting > KVM_HC_MAP_GPA_RANGE hypercall, SEV/SEV-ES guest should communicate private/shared > pages to the hypervisor, this information can be used to mark page shared/private. One concern here may be that the VMM doesn't know which guests have KVM_FEATURE_HC_MAP_GPA_RANGE support and which don't. Only once the guest boots does the guest tell KVM that it supports KVM_FEATURE_HC_MAP_GPA_RANGE. If the guest doesn't we need to pin all the memory before we run the guest to be safe to be safe.
