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
