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From:   "Kalra, Ashish" <Ashish.Kalra@amd.com>
To:     Peter Gonda <pgonda@google.com>
CC:     the arch/x86 maintainers <x86@kernel.org>,
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Subject: RE: [PATCH Part2 v6 28/49] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_FINISH
 command
Thread-Topic: [PATCH Part2 v6 28/49] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_FINISH
 command
Thread-Index: AQHYlS9F8Lv7wrs5NE6SmQ1dqRz5cK15oJ+wgAExBACAAAQscA==
Date:   Tue, 12 Jul 2022 15:22:07 +0000
Message-ID: <SN6PR12MB2767D8C552388D438D9F88268E869@SN6PR12MB2767.namprd12.prod.outlook.com>
References: <cover.1655761627.git.ashish.kalra@amd.com>
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[AMD Official Use Only - General]

Hello Peter,

>> >Given the guest uses the SNP NAE AP boot protocol we were expecting tha=
t there would be some option to add vCPUs to the VM but mark them as "pendi=
ng 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 possib=
le to add a new add an argument to KVM_SNP_LAUNCH_FINISH to tell it which v=
CPUs to LAUNCH_UPDATE VMSA pages for or similarly a new argument for KVM_CR=
EATE_VCPU?
>>
>> But don't we want/need to measure all vCPUs using LAUNCH_UPDATE_VMSA bef=
ore 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_L=
AUNCH_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 num=
bers of vCPUs to have different launch digests. Its my understanding the SN=
P AP >Creation protocol was to solve this so that VMs with different vcpu c=
ounts have the same launch digest.

>Looking at patch "[Part2,v6,44/49] KVM: SVM: Support SEV-SNP AP Creation N=
AE 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. I=
nstead 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 runna=
ble. I believe this is all >setup in the UEFI patch:
>https://nam11.safelinks.protection.outlook.com/?url=3Dhttps%3A%2F%2Fwww.ma=
il-archive.com%2Fdevel%40edk2.groups.io%2Fmsg38460.html&amp;data=3D05%7C01%=
7CAshish.Kalra%40amd.com%7Ca40178ac6f284a9e33aa08da64152baa%>7C3dd8961fe488=
4e608e11a82d994e183d%7C0%7C0%7C637932339382401133%7CUnknown%7CTWFpbGZsb3d8e=
yJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C=
%7C%7C&amp;sdata=3DZaiHHo9S24f9BB6E%>2FjexOt5TdKJQXxQDJI5QoYdDDHc%3D&amp;re=
served=3D0.

Yes, I discussed the same with Tom, and this will be supported going forwar=
d, only the BSP will need to go through the LAUNCH_UPDATE_VMSA and at runti=
me 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 c=
ount argument for SNP_LAUNCH_FINISH, won't the statically created vCPUs in =
kvm->created_vcpus/online_vcpus be sufficient for that, any dynamically cre=
ated
vCPU's won't be part of the initial measurement or LaunchDigest of the VM, =
right ?

Thanks,
Ashish