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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 06, 2020 at 01:37:08PM +0100, 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?
> 
> Stefan


Backends such as vhost-user can handle > iov max. Devices such
as scsi and block have a limit for s/g separate from vq size.

-- 
MST