[Qemu-devel] Re: [PATCH] qemu/virtio: move features to an inline function
Michael S. Tsirkin
mst at redhat.com
Tue Aug 11 09:25:37 PDT 2009
On Tue, Aug 11, 2009 at 11:08:32AM -0500, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
>>> Let's say we supported virtio-vbus along with virtio-pci. What does
>>> virtio_blk_get_features() do to mask out sg_indirect? For all
>>> virtio-blk knows, it could be on top of virtio-vbus.
>>>
>>
>> So? VIRTIO_RING_F_INDIRECT_DESC applies to all transports.
>> Just clear this bit.
>>
>
> You can have many layers with virtio. device + transport + ring
>
> virtio-vbus would have a different transport and a different ring
> implementation. So no, VIRTIO_RING_F_INDIRECT_DESC wouldn't apply to
> virtio-vbus.
>
>>>> This would break things like
>>>> migrating between userspace and kernel virtio (something that I
>>>> support).
>>>>
>>> The PIT uses a common state structure and common code for
>>> save/restore. This makes migration compatible.
>>>
>>
>> Isn't device name put in the machine config, which presumably is
>> send along as well?
>>
>
> Good question. I don't know the best way to resolve this.
>
> Maybe migration between devices isn't such a good idea. It's
> conceivable that vhost will require some state that isn't present in the
> userspace virtio-net.
It can't. It switches to userspace before migration.
> I think this requires some thought.
>
>>> In this case, it's two separate implementations of the same device.
>>> I think it makes sense for them to be separate devices.
>>>
>>> Regards,
>>>
>>> Anthony Liguori
>>>
>>
>> Hmm, I see what you mean. But kernel virtio is harder. Unlike
>> PIT/APIC, it is not a separate codepath. It still needs
>> all of userspace virtio to support live migration and non-MSI guests.
>> Really, it's the same device that switches between kernel and userspace
>> modes on the fly.
>>
>> This will become clearer from code when I implement migration for vhost,
>> but basically you switch to userspace when you start migration, and
>> back to kernel if migration fails. You also switch to kernel when MSI
>> is enabled and back to userspace when it is disabled.
>>
>
> Why bother switching to userspace for migration? Can't you just have
> get/set ioctls for the state?
I have these. But live migration requires dirty page logging.
I do not want to implement it in kernel.
> Regards,
>
> Anthony Liguori
>
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