Re: [Qemu-devel] [PATCH v7 1/8] virtio: endian-ambivalent targets using legacy virtio

[Cedric Le Goater]

On 04/14/2014 02:49 PM, Alexander Graf wrote:
> 
> On 14.04.14 14:46, Michael S. Tsirkin wrote:
>> On Mon, Apr 14, 2014 at 02:40:04PM +0200, Alexander Graf wrote:
>>> On 14.04.14 14:37, Michael S. Tsirkin wrote:
>>>> On Mon, Apr 14, 2014 at 02:29:20PM +0200, Alexander Graf wrote:
>>>>> On 14.04.14 14:24, Michael S. Tsirkin wrote:
>>>>>
>>>> This will have to be measured and proved by whoever's proposing the
>>>> patch, not by reviewers.  Platforms such as AMD which don't do
>>>> prediction well would be especially interesting to test on.
>>> Sure, Greg, can you do that? I'm sure Michael has test cases
>>> available he can give you to measure performance on this.
>>>
>>> Speaking of which, how does all of this work with vhost?
>>>
>>>
>>> Alex
>> I think that's missing.
>> As a first step, we need to disable vhost when
>> host/guest endian-ness do not match.
>>
>> We could also add cross-endian support to vhost.
>>
>> Or just wait a couple more months for virtio 1.0 which is fixed
>> endian-ness.
> 
> That won't help for current ppc64le guests, so I suppose we will 
> need cross-endian vhost.

Yes. For the moment, vhost=off is needed and even with that, qemu needs 
to byteswap a few attributes (see virtio-net header in patch 3). I am 
looking for a simple way to propagate the vring endianness to the host 
without breaking the compatibility with the current virtio drivers. 
Hopefully this is feasible. 


C.