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SubjectRe: [RFC] KVM: x86: Support KVM VMs sharing SEV context
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On 3/11/21 11:29 AM, Paolo Bonzini wrote:

> 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.

We've been starting up our Migration Handler via OVMF. I'm not sure if
this would work with a minimal setup in QEMU.

-Tobin

>
> 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
>

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