Re: [RFC v2 2/3] libvhost-user: handle endianness as mandated by the spec

[Marc Hartmayer <mhartmay@linux.ibm.com>]

On Wed, Jul 29, 2020 at 10:13 AM -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Tue, Jul 28, 2020 at 12:52:11PM +0200, Marc Hartmayer wrote:
>> On Tue, Jul 21, 2020 at 06:44 PM +0200, Halil Pasic <pasic@linux.ibm.com> wrote:
>> > On Tue, 21 Jul 2020 09:40:10 -0400
>> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> >
>> >> On Fri, Jul 17, 2020 at 11:29:28AM +0200, Marc Hartmayer 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]. We will fence non-legacy virtio devices with the upcoming patch.
>> >> > 
>> >> > [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>
>> >> > 
>> >> > ---
>> >> > Note: As we don't support legacy virtio devices  
>> >> 
>> >> Who's "we" in this sentence? vhost user supports legacy generally ...
>> >
>> > In that sentence "we" is the library "libvhost-user". I would like
>> > to avoid s390x being an oddball regarding this. Marc's previous
>> > version made an attempt at correct endianness handling for legacy
>> > and non-legacy. That spawned a discussion on how we don't want
>> > legacy devices in this context. This series makes what seemed to be the
>> > consensus reached in that discussion explicit: namely that libvhost-user
>> > does not support legacy-virtio.
>> 
>> Hi Michael,
>> 
>> Polite ping - what’s your opinion? Thanks in advance.
>> 
>> […snip]
>
> It's a reasonable limitation, but I also don't see anything
> that verifies that device is modern only.
> E.g. fail setting features if VIRTIO_F_VERSION_1 is not there?

That’s implemented in patch 3 to emphasize how the fencing is done. For
the next series I’ll merge patch 2 and 3.

>
>
>> -- 
>> Kind regards / Beste Grüße
>>    Marc Hartmayer
>> 
>> IBM Deutschland Research & Development GmbH
>> Vorsitzender des Aufsichtsrats: Gregor Pillen 
>> Geschäftsführung: Dirk Wittkopp
>> Sitz der Gesellschaft: Böblingen
>> Registergericht: Amtsgericht Stuttgart, HRB 243294
>
-- 
Kind regards / Beste Grüße
   Marc Hartmayer

IBM Deutschland Research & Development GmbH
Vorsitzender des Aufsichtsrats: Gregor Pillen 
Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen
Registergericht: Amtsgericht Stuttgart, HRB 243294