Re: [PATCH v2 3/3] tun: Set num_buffers for virtio 1.0

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/01/16 10:06, Jason Wang wrote:
> On Wed, Jan 15, 2025 at 1:07 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>
>> On 2025/01/13 12:04, Jason Wang wrote:
>>> On Fri, Jan 10, 2025 at 7:12 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>>>
>>>> On 2025/01/10 19:23, Michael S. Tsirkin wrote:
>>>>> On Fri, Jan 10, 2025 at 11:27:13AM +0800, Jason Wang wrote:
>>>>>> On Thu, Jan 9, 2025 at 2:59 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>>>>>>
>>>>>>> The specification says the device MUST set num_buffers to 1 if
>>>>>>> VIRTIO_NET_F_MRG_RXBUF has not been negotiated.
>>>>>>
>>>>>> Have we agreed on how to fix the spec or not?
>>>>>>
>>>>>> As I replied in the spec patch, if we just remove this "MUST", it
>>>>>> looks like we are all fine?
>>>>>>
>>>>>> Thanks
>>>>>
>>>>> We should replace MUST with SHOULD but it is not all fine,
>>>>> ignoring SHOULD is a quality of implementation issue.
>>>>>
>>>
>>> So is this something that the driver should notice?
>>>
>>>>
>>>> Should we really replace it? It would mean that a driver conformant with
>>>> the current specification may not be compatible with a device conformant
>>>> with the future specification.
>>>
>>> I don't get this. We are talking about devices and we want to relax so
>>> it should compatibile.
>>
>>
>> The problem is:
>> 1) On the device side, the num_buffers can be left uninitialized due to bugs
>> 2) On the driver side, the specification allows assuming the num_buffers
>> is set to one.
>>
>> Relaxing the device requirement will replace "due to bugs" with
>> "according to the specification" in 1). It still contradicts with 2) so
>> does not fix compatibility.
> 
> Just to clarify I meant we can simply remove the following:
> 
> """
> The device MUST use only a single descriptor if VIRTIO_NET_F_MRG_RXBUF
> was not negotiated. Note: This means that num_buffers will always be 1
> if VIRTIO_NET_F_MRG_RXBUF is not negotiated.
> """
> 
> And
> 
> """
> If VIRTIO_NET_F_MRG_RXBUF has not been negotiated, the device MUST set
> num_buffers to 1.
> """
> 
> This seems easier as it reflects the fact where some devices don't set
> it. And it eases the transitional device as it doesn't need to have
> any special care.

That can potentially break existing drivers that are compliant with the 
current and assumes the num_buffers is set to 1.

Regards,
Akihiko Odaki

> 
> Then we don't need any driver normative so I don't see any conflict.
> 
> Michael suggests we use "SHOULD", but if this is something that the
> driver needs to be aware of I don't know how "SHOULD" can help a lot
> or not.
> 
>>
>> Instead, we should make the driver requirement stricter to change 2).
>> That is what "[PATCH v3] virtio-net: Ignore num_buffers when unused" does:
>> https://lore.kernel.org/r/20250110-reserved-v3-1-2ade0a5d2090@daynix.com
>>
>>>
>>>>
>>>> We are going to fix all implementations known to buggy (QEMU and Linux)
>>>> anyway so I think it's just fine to leave that part of specification as is.
>>>
>>> I don't think we can fix it all.
>>
>> It essentially only requires storing 16 bits. There are details we need
>> to work out, but it should be possible to fix.
> 
> I meant it's not realistic to fix all the hypervisors. Note that
> modern devices have been implemented for about a decade so we may have
> too many versions of various hypervisors. (E.g DPDK seems to stick
> with the same behaviour of the current kernel).
 > >>
>> Regards,
>> Akihiko Odaki
>>
> 
> Thanks
>