Re: [Qemu-devel] [virtio-dev] Re: [virtio-dev] Re: [PATCH v1] virtio-net: enable configurable tx queue size

[Jason Wang <jasowang@redhat.com>]

On 2017年06月13日 14:08, Wei Wang wrote:
> On 06/13/2017 11:55 AM, Jason Wang wrote:
>>
>> On 2017年06月13日 11:51, Wei Wang wrote:
>>> On 06/13/2017 11:19 AM, Jason Wang wrote:
>>>>
>>>> On 2017年06月13日 11:10, Wei Wang wrote:
>>>>> On 06/13/2017 04:43 AM, Michael S. Tsirkin wrote:
>>>>>> On Mon, Jun 12, 2017 at 05:30:46PM +0800, Wei Wang wrote:
>>>>>>> Ping for comments, thanks.
>>>>>> This was only posted a week ago, might be a bit too short for some
>>>>>> people.
>>>>> OK, sorry for the push.
>>>>>> A couple of weeks is more reasonable before you ping.  Also, I
>>>>>> sent a bunch of comments on Thu, 8 Jun 2017.  You should probably
>>>>>> address these.
>>>>>>
>>>>> I responded to the comments. The main question is that I'm not sure
>>>>> why we need the vhost backend to support VIRTIO_F_MAX_CHAIN_SIZE.
>>>>> IMHO, that should be a feature proposed to solve the possible issue
>>>>> caused by the QEMU implemented backend.
>>>> The issue is what if there's a mismatch of max #sgs between qemu and
>>>> vhost?
>>>>
>>> When the vhost backend is used, QEMU is not involved in the data path.
>>> The vhost backend
>>> directly gets what is offered by the guest from the vq. Why would
>>> there be a mismatch of
>>> max #sgs between QEMU and vhost, and what is the QEMU side max #sgs
>>> used for? Thanks.
>> You need query the backend max #sgs in this case at least. no? If not
>> how do you know the value is supported by the backend?
>>
>> Thanks
>>
> Here is my thought: vhost backend has already been supporting 1024 sgs,
> so I think it might not be necessary to query the max sgs that the vhost
> backend supports. In the setup phase, when QEMU detects the backend is
> vhost, it assumes 1024 max sgs is supported, instead of giving an extra
> call to query.

We can probably assume vhost kernel supports up to 1024 sgs. But how 
about for other vhost-user backends?

And what you said here makes me ask one of my questions in the past:

Do we have plan to extend 1024 to a larger value or 1024 looks good for 
the future years? If we only care about 1024, there's even no need for a 
new config filed, a feature flag is more than enough. If we want to 
extend it to e.g 2048, we definitely need to query vhost backend's limit 
(even for vhost-kernel).

Thanks

>
> The advantage is that people who is using the vhost backend can upgrade
> to use 1024 tx queue size by only applying the QEMU patches. Adding an 
> extra
> call to query the size would need to patch their vhost backend (like 
> vhost-user),
> which is difficult for them.
>
>
> Best,
> Wei
>
>
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