Re: [PATCH Part2 v6 28/49] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_FINISH command

[Peter Gonda]

On Tue, Jul 12, 2022 at 9:22 AM Kalra, Ashish <Ashish.Kalra@xxxxxxx> wrote:
>
> [AMD Official Use Only - General]
>
> Hello Peter,
>
> >> >Given the guest uses the SNP NAE AP boot protocol we were expecting that there would be some option to add vCPUs to the VM but mark them as "pending AP boot creation protocol" state. This would allow the LaunchDigest of a VM doesn't change >just because its vCPU count changes. Would it be possible to add a new add an argument to KVM_SNP_LAUNCH_FINISH to tell it which vCPUs to LAUNCH_UPDATE VMSA pages for or similarly a new argument for KVM_CREATE_VCPU?
> >>
> >> But don't we want/need to measure all vCPUs using LAUNCH_UPDATE_VMSA before we issue SNP_LAUNCH_FINISH command ?
> >>
> >> If we are going to add vCPUs and mark them as "pending AP boot creation" state then how are we going to do LAUNCH_UPDATE_VMSAs for them after SNP_LAUNCH_FINISH ?
>
> >If I understand correctly we don't need or even want the APs to be LAUNCH_UPDATE_VMSA'd. LAUNCH_UPDATEing all the VMSAs causes VMs with different numbers of vCPUs to have different launch digests. Its my understanding the SNP AP >Creation protocol was to solve this so that VMs with different vcpu counts have the same launch digest.
>
> >Looking at patch "[Part2,v6,44/49] KVM: SVM: Support SEV-SNP AP Creation NAE event" and section "4.1.9 SNP AP Creation" of the GHCB spec. There is no need to mark the LAUNCH_UPDATE the AP's VMSA or mark the vCPUs runnable. Instead we >can do that only for the BSP. Then in the guest UEFI the BSP can: create new VMSAs from guest pages, RMPADJUST them into the RMP state VMSA, then use the SNP AP Creation NAE to get the hypervisor to mark them runnable. I believe this is all >setup in the UEFI patch:
> >https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.mail-archive.com%2Fdevel%40edk2.groups.io%2Fmsg38460.html&amp;data=05%7C01%7CAshish.Kalra%40amd.com%7Ca40178ac6f284a9e33aa08da64152baa%>7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637932339382401133%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=ZaiHHo9S24f9BB6E%>2FjexOt5TdKJQXxQDJI5QoYdDDHc%3D&amp;reserved=0.
>
> Yes, I discussed the same with Tom, and this will be supported going forward, only the BSP will need to go through the LAUNCH_UPDATE_VMSA and at runtime the guest can dynamically create more APs using the SNP AP Creation NAE event.
>
> Now, coming back to the original question, why do we need a separate vCPU count argument for SNP_LAUNCH_FINISH, won't the statically created vCPUs in kvm->created_vcpus/online_vcpus be sufficient for that, any dynamically created
> vCPU's won't be part of the initial measurement or LaunchDigest of the VM, right ?

Are you suggesting that QEMU will KVM_CREATE_VCPU the BSP, then
LAUNCH_FINISH, then KVM_CREATE_VCPU all the APs to their VMSAs were
not LAUNCH_UPDATED? If so, it seems annoying to have to create vCPUs
at different times to get their VMSAs into different states. That's
why I was suggesting some other mechanism so we can continue to
KVM_CREATE_VCPU all the vCPUs at the same time.

>
> Thanks,
> Ashish