linux-kernel - Re: [RFC v2] KVM: x86: Support KVM VMs sharing SEV context

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Date:   Fri, 9 Apr 2021 10:14:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     jejb@...ux.ibm.com, Steve Rutherford <srutherford@...gle.com>
Cc:     Ashish Kalra <ashish.kalra@....com>,
        Nathan Tempelman <natet@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        dovmurik@...ux.vnet.ibm.com, lersek@...hat.com, frankeh@...ibm.com
Subject: Re: [RFC v2] KVM: x86: Support KVM VMs sharing SEV context

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?).

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.