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To:     Tobin Feldman-Fitzthum <tobin@linux.ibm.com>, natet@google.com
Cc:     Dov Murik <dovmurik@linux.vnet.ibm.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, x86@kernel.org,
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        Ashish Kalra <ashish.kalra@amd.com>,
        Laszlo Ersek <lersek@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        Hubertus Franke <frankeh@us.ibm.com>
References: <20210224085915.28751-1-natet@google.com>
 <7829472d-741c-1057-c61f-321fcfb5bdcd@linux.ibm.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context
Message-ID: <35dde628-f1a8-c3bf-9c7d-7789166b0ee1@redhat.com>
Date:   Thu, 11 Mar 2021 17:29:35 +0100
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On 11/03/21 16:30, Tobin Feldman-Fitzthum wrote:
> I am not sure how the mirror VM will be supported in QEMU. Usually there 
> is one QEMU process per-vm. Now we would need to run a second VM and 
> communicate with it during migration. Is there a way to do this without 
> adding significant complexity?

I can answer this part.  I think this will actually be simpler than with 
auxiliary vCPUs.  There will be a separate pair of VM+vCPU file 
descriptors within the same QEMU process, and some code to set up the 
memory map using KVM_SET_USER_MEMORY_REGION.

However, the code to run this VM will be very small as the VM does not 
have to do MMIO, interrupts, live migration (of itself), etc.  It just 
starts up and communicates with QEMU using a mailbox at a predetermined 
address.

I also think (but I'm not 100% sure) that the auxiliary VM does not have 
to watch changes in the primary VM's memory map (e.g. mapping and 
unmapping of BARs).  In QEMU terms, the auxiliary VM's memory map tracks 
RAMBlocks, not MemoryRegions, which makes things much simpler.

There are already many examples of mini VMMs running special purpose VMs 
in the kernel's tools/testing/selftests/kvm directory, and I don't think 
the QEMU code would be any more complex than that.

Paolo