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

[Stefan Hajnoczi]

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?

Stefan

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