Re: [PATCH 2/2] libvhost-user: handle endianness as mandated by the spec

[Marc Hartmayer]

On Mon, Aug 03, 2020 at 11:26 AM +0200, Cornelia Huck <cohuck@redhat.com> wrote:
> On Thu, 30 Jul 2020 16:07:31 +0200
> Marc Hartmayer <mhartmay@linux.ibm.com> wrote:
>
>> Since virtio existed even before it got standardized, the virtio
>> standard defines the following types of virtio devices:
>> 
>>  + legacy device (pre-virtio 1.0)
>>  + non-legacy or VIRTIO 1.0 device
>>  + transitional device (which can act both as legacy and non-legacy)
>> 
>> Virtio 1.0 defines the fields of the virtqueues as little endian,
>> while legacy uses guest's native endian [1]. Currently libvhost-user
>> does not handle virtio endianness at all, i.e. it works only if the
>> native endianness matches with whatever is actually needed. That means
>> things break spectacularly on big-endian targets. Let us handle virtio
>> endianness for non-legacy as required by the virtio specification
>> [1]. 
>
> Maybe add
>
> "and fence legacy virtio, as there is no safe way to figure out the
> needed endianness conversions for all cases."

Okay.

>
>> The fencing of legacy virtio devices is done in
>> `vu_set_features_exec`.
>
> Not that I disagree with fencing legacy virtio, but looking at some
> vhost-user* drivers, I'm not sure everything will work as desired for
> those (I might be missing something, though.)
>
> - vhost-user-blk lists VERSION_1 in the supported features, but
>   vhost-user-scsi doesn't... is there some inheritance going on that
>   I'm missing?
> - vhost-user-gpu-pci inherits from virtio-gpu-pci, so I guess it's fine
> - vhost-user-input should also always have been virtio-1
>
> So, has anybody been using vhost-user-scsi and can confirm that it
> still works, or at least can be made to work?

Unfortunately, I don’t have the required hardware :/ Can please anybody
verify this?

>
>> 
>> [1] 
>> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-210003
>> 
>> Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
>> ---
>>  contrib/libvhost-user/libvhost-user.c | 77 +++++++++++++++------------
>>  1 file changed, 43 insertions(+), 34 deletions(-)
>
> The code change per se LGTM.

Thanks for the feedback!

>
-- 
Kind regards / Beste Grüße
   Marc Hartmayer

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