Re: Any reason VIRTQUEUE_MAX_SIZE is 1024? Can we increase this limit?

[Jason Wang <jasowang@redhat.com>]

On 2020/8/6 下午8:37, Stefan Hajnoczi wrote:
> On Wed, Aug 05, 2020 at 08:13:29AM -0400, Michael S. Tsirkin wrote:
>> On Wed, Aug 05, 2020 at 01:11:07PM +0100, Stefan Hajnoczi wrote:
>>> On Thu, Jul 30, 2020 at 07:46:09AM +0000, Yajun Wu wrote:
>>>> I'm doing iperf test on VIRTIO net through vhost-user(HW VDPA).
>>>> Find maximal acceptable tx_queue_size/rx_queue_size is 1024.
>>>> Basically increase queue size can get better RX rate for my case.
>>>>
>>>> Can we increase the limit(VIRTQUEUE_MAX_SIZE) to 8192 to possibly gain better performance?
>>> Hi,
>>> The VIRTIO 1.1 specification says the maximum number of descriptors is
>>> 32768 for both split and packed virtqueues.
>>>
>>> The vhost kernel code seems to support 32768.
>>>
>>> The 1024 limit is an implementation limit in QEMU. Increasing it would
>>> require QEMU code changes. For example, VIRTQUEUE_MAX_SIZE is used as
>>> the size of arrays.
>>>
>>> I can't think of a fundamental reason why QEMU needs to limit itself to
>>> 1024 descriptors. Raising the limit would require fixing up the code and
>>> ensuring that live migration remains compatible with older versions of
>>> QEMU.
>>>
>>> Stefan
>> There's actually a reason for a limit: in theory the vq size
>> also sets a limit on the number of scatter/gather entries.
>> both QEMU and vhost can't handle a packet split over > 1k chunks.
>>
>> We could add an extra limit for s/g size like block and scsi do,
>> this will need spec, guest and host side work.
> Interesting, thanks for explaining! This could be made explicit by
> changing the QEMU code to:
>
> include/hw/virtio/virtio.h:#define VIRTQUEUE_MAX_SIZE IOV_MAX
>
> Looking more closely at the vhost kernel code I see that UIO_MAXIOV is
> used in some places but not in vhost_vring_set_num() (ioctl
> VHOST_SET_VRING_NUM). Is there a reason why UIO_MAXIOV isn't enforced
> when the application sets the queue size?


Actually three things:

1) queue size
2) #descriptors in a list
3) IOV size

Spec limit the 2) to 1) but 2) may not equal to 3).

So enforcing UIO_MAXIOV can not solve the problem completely.

For vhost-net, it depends on socket to build skb which requires an iov 
array to work. We need remove this limitation by:

- build skb by vhost-net itself
- do piecewise copying

Then we're not limited with #iov and more and support up to what spec 
supports.

Thanks


>
> Stefan