Re: [Qemu-devel] [PATCH v4] Add machine parameter qemu-kvm-migration for live migrate compatibility with qemu-kvm

[Alex Bligh <alex@alex.org.uk>]

Markus,

On 24 Sep 2014, at 09:05, Markus Armbruster <armbru@redhat.com> wrote:

> Alex Bligh <alex@alex.org.uk> writes:
> 
>> This patch series adds inbound migrate capability from qemu-kvm version
>> 1.0. The main ideas are those set out in Cole Robinson's patch here:
>> http://pkgs.fedoraproject.org/cgit/qemu.git/tree/0001-Fix-migration-from-qemu-kvm.patch?h=f20
>> however, rather than patching statically (and breaking inbound
>> migration on existing machine types), I have added a new machine
>> parameter (qemu-kvm-migration) which when turned on affects the pc-1.0
>> machine type. Usage:
>>        -machine pc-1.0,qemu-kvm-migration=on
> 
> Forgive me if this has been discussed already: why not simply a separate
> machine type "pc-1.0-qemu-kvm"?

That's what v2 of the patch set does (and I prefer v2). However, mst
wanted it done this way.

> 
>> Three aproaches are taken:
>> 
>> * cirrus-vga.vgamem_mb defaults to 16 rather than 8. In	order to
>>  keep -global cirrus-vga.vgamem_mb working even with
>>  qemu-kvm-migration=on, this is monkey-patched	 into the default
>>  value			 of the MachineState structure's  compat_props list.
> 
> This part fires only for pc-1.0, because it's in
> pc_early_init_pci_1_0().

Yes, intentional. I should have noted that. That's because
qemu-kvm-migration=on the VRAM change was designed to
work only with pc-1.0. I haven't looked at trying to make
other (older) qemu-kvm machine migrations work, but (with the stuff
below), I would guess it would work with the appropriate
command line options for VRAM size.

Obviously I could put early init elsewhere.

>> * In hw/timer/i8254_common.c, the VMSTATE_UINT32_TEST macro
>>  is used to test the version for the irq_disable flags,
>>  allowing version 3 or more, or version 2 for an inbound
>>  migrate from qemu-kvm (only).
>> 
>> * In hw/acpi/piix4.c, qemu-kvm incorrectly uses version 2 for
>>  a version 3 structure, causing acpi_load_old to be used.
>>  acpi_load_old detects this situation based on the machine type
>>  and restarts the attempt to load the vmstate using a
>>  customised VMStateDescription. The above cleaner approach is
>>  unavailable here.
> 
> These parts apply to all machine types, don't they?

They apply only when qemu-kvm-migration=on is selected, but to
any machine type; however machine types newer than pc-1.0 will
be exporting v3 I think anyway.

>> Changes since v1:
>> * Do not use a machine type, use a machine parameter.
> 
> Okay, it has been discussed already.  I'd appreciate a brief recap all
> the same.

See above. I preferred the machine type. But I got to learn more about
QOM on the way :-)

-- 
Alex Bligh