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From:   Peter Gonda <pgonda@google.com>
Date:   Tue, 12 Jul 2022 08:45:18 -0600
Message-ID: <CAMkAt6ryLr6a5iQnwZQT3hqwEpZpb7bn-T8SDY6=5zYs_5NBow@mail.gmail.com>
Subject: Re: [PATCH Part2 v6 28/49] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_FINISH command
To:     "Kalra, Ashish" <Ashish.Kalra@amd.com>
Cc:     "the arch/x86 maintainers" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
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On Mon, Jul 11, 2022 at 4:41 PM Kalra, Ashish <Ashish.Kalra@amd.com> wrote:
>
> [AMD Official Use Only - General]
>
> Hello Peter,
>
> >> The KVM_SEV_SNP_LAUNCH_FINISH finalize the cryptographic digest and
> >> stores it as the measurement of the guest at launch.
> >>
> >> While finalizing the launch flow, it also issues the LAUNCH_UPDATE
> >> command to encrypt the VMSA pages.
>
> >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 "pendin=
g 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 possibl=
e to add a new add an argument to KVM_SNP_LAUNCH_FINISH to tell it which vC=
PUs to LAUNCH_UPDATE VMSA pages for or similarly a new argument for KVM_CRE=
ATE_VCPU?
>
> But don't we want/need to measure all vCPUs using LAUNCH_UPDATE_VMSA befo=
re 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_LA=
UNCH_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://www.mail-archive.com/devel@edk2.groups.io/msg38460.html.