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From:   Steve Rutherford <srutherford@google.com>
Date:   Fri, 9 Apr 2021 13:24:06 -0700
Message-ID: <CABayD+dCiuXXD=erOuVFEZOULrDvdqOTyHSA4drHArvBicTzcw@mail.gmail.com>
Subject: Re: [RFC v2] KVM: x86: Support KVM VMs sharing SEV context
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     jejb@linux.ibm.com, Ashish Kalra <ashish.kalra@amd.com>,
        Nathan Tempelman <natet@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        X86 ML <x86@kernel.org>, KVM list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        David Rientjes <rientjes@google.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        dovmurik@linux.vnet.ibm.com, lersek@redhat.com, frankeh@us.ibm.com
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On Fri, Apr 9, 2021 at 1:14 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 09/04/21 03:18, James Bottomley wrote:
> > If you want to share ASIDs you have to share the firmware that the
> > running VM has been attested to.  Once the VM moves from LAUNCH to
> > RUNNING, the PSP won't allow the VMM to inject any more firmware or do
> > any more attestations.
>
> I think Steve is suggesting to just change the RIP of the mirror VM,
> which would work for SEV but not SEV-ES (the RAM migration helper won't
> *suffice* for SEV-ES, but perhaps you could use the PSP to migrate the
> VMSA and the migration helper for the rest?).
Exactly: you can use the existing PSP flows to migrate both the
migration helper itself and the necessary VMSAs.
>
> If you want to use a single firmware binary, SEC does almost no I/O
> accesses (the exception being the library constructor from
> SourceLevelDebugPkg's SecPeiDebugAgentLib), so you probably can:
>
> - detect the migration helper hardware in PlatformPei, either from
> fw_cfg or based on the lack of it
>
> - either divert execution to the migration helper through
> gEfiEndOfPeiSignalPpiGuid, or if it's too late add a new boot mode and
> PPI to DxeLoadCore.
>
> Paolo
>
> > What you mirror after this point can thus only
> > contain what has already been measured or what the guest added.  This
> > is why we think there has to be a new entry path into the VM for the
> > mirror vCPU.
>