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Subject: Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context
To:     Paolo Bonzini <pbonzini@redhat.com>, natet@google.com
Cc:     Dov Murik <dovmurik@linux.vnet.ibm.com>,
        Tom Lendacky <thomas.lendacky@amd.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        srutherford@google.com, seanjc@google.com, rientjes@google.com,
        Brijesh Singh <brijesh.singh@amd.com>,
        Ashish Kalra <ashish.kalra@amd.com>,
        Laszlo Ersek <lersek@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        Hubertus Franke <frankeh@us.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>
References: <20210224085915.28751-1-natet@google.com>
 <7829472d-741c-1057-c61f-321fcfb5bdcd@linux.ibm.com>
 <35dde628-f1a8-c3bf-9c7d-7789166b0ee1@redhat.com>
From:   Tobin Feldman-Fitzthum <tobin@linux.ibm.com>
Message-ID: <adb84c91-1651-94b6-0084-f86296e96530@linux.ibm.com>
Date:   Mon, 15 Mar 2021 13:05:16 -0400
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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
>