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

[Wei Wang <wei.w.wang@intel.com>]

On 06/13/2017 02:29 PM, Jason Wang wrote:
> The issue is what if there's a mismatch of max #sgs between qemu and
>>>> 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?
>
So far, I haven't seen any vhost backend implementation supporting less 
than 1024 sgs.


> 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).
>

According to virtio spec (e.g. 2.4.4), unreasonably large descriptors are
not encouraged to be used by the guest. If possible, I would suggest to use
1024 as the largest number of descriptors that the guest can chain, even 
when
we have larger queue size in the future. That is,
if (backend == QEMU backend)
     config.max_chain_size = 1023 (defined by the qemu backend 
implementation);
else if (backend == vhost)
     config.max_chain_size = 1024;

It is transparent to the guest. From the guest's point of view, all it 
knows is a value
given to him via reading config.max_chain_size.


Best,
Wei