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Date: Wed, 19 Mar 2014 16:52:16 +0100
From: "Serge E. Hallyn" <serge@hallyn.com>
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Subject: Re: [Qemu-devel] migrating instances from qemu-kvm to qemu
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, "Serge E. Hallyn" <serge@hallyn.com>

Quoting Paolo Bonzini (pbonzini@redhat.com):
> Il 19/03/2014 16:42, Serge E. Hallyn ha scritto:
> >Hi,
> >
> >at https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1291321
> >it was found that migrating running vms from a machine with
> >qemu-kvm to one with qemu, migration fails due to some mismatches.
> >The first one we usually hit is
> >
> >Length mismatch: vga.vram: 1000000 in != 800000
> >
> >while the second one is network card (and I have not gone beyond
> >that).
> 
> This is just a different default.  Make sure you specify the right
> model on the command line.
> 
> >The vga one can be handled on the command line by
> >specifying the -global cirrus-vga.vrammem_mb=8.  However that
> >doesn't help with a libvirt migration.
> 
> You can patch Ubuntu's QEMU to detect old machine types (pc-1.2 and
> earlier) and give cirrus-vga 16MB memory by default.  Migration only
> works with versioned machine types (or between same-version QEMU),
> so it's okay to only do it there.
> 
> >I guess this happens at ram_load() unfortunately - is there any
> >good way that this could be detected at incoming migration time
> >and the virtual hardware modified as needed for migration to
> >continue?
> 
> It cannot, but the destination can be patched to operate correctly
> for the old machine types, on the assumption that migration from the
> old machine types is always from Ubuntu's qemu-kvm.
> 
> Paolo

Ah, thank you, that assumption should be safe and sounds like a
great idea.

thanks,
-serge