Re: [Qemu-devel] [PATCH 2/4] s390-virtio-bus: move common virtio properties to virtio s390 device class

[Ming Lei <ming.lei@canonical.com>]

On Tue, Jun 17, 2014 at 6:25 PM, Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> On Tue, 17 Jun 2014 18:15:39 +0800
> Ming Lei <ming.lei@canonical.com> wrote:
>
>> On Tue, Jun 17, 2014 at 4:46 PM, Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
>> >>
>> >> Looks like s390 virtio-blk never enables the two common features, is
>> >> there any reason the two features can't be supported by s390?
>> >
>> > Indirect descriptors are fine. event_idx will not work IIUC because we
>> > always need to do a sync before we see changes, and this needs an
>> > interrupt to trigger.
>>
>> Sounds like the old s390 isn't cache coherent? Because you mean
>> write in one side can only be observed from another side with an
>> explicit notification or interrupt.
>>
>> On arm/arm64, we didn't see any problem with event_idx.
>
> But you probably have the queues in guest memory, as on other
> transports (including virtio-ccw)? The old s390-virtio transport keeps
> the devices and their virtqueues in a memory area behind the guest
> memory - the guest does not see that memory directly, but a sync has to
> be performed to see virtqueue movement (see s390_virtio_device_sync()).

OK, it looks like a real physical device, :-)

I will keep s390-virtio as it is, thanks for your explanation.


Thanks,
--
Ming Lei