Re: [Qemu-devel] value of VIRTQUEUE_MAX_SIZE

[Stefan Hajnoczi]

On Thu, Feb 05, 2015 at 03:29:17PM +0100, Peter Lieven wrote:
> Am 05.02.2015 um 15:00 schrieb Stefan Hajnoczi:
> >On Fri, Jan 30, 2015 at 10:08:02PM +0100, Peter Lieven wrote:
> >>Just wondering if VIRTQUEUE_MAX_SIZE in include/hw/virtio/virtio.h should 
> >>not be equal to IOV_MAX instead of the hardcoded 1024?
> >The vring queue size is guest-visible to some extent (e.g. vring memory
> >layout).  Tying it to a constant that is defined by the host operating
> >system could lead to problems (e.g. live migration between different
> >hosts).
> >
> >Anyway, all of the virtio devices have a queue size that is less than or
> >equal to VIRTQUEUE_MAX_SIZE (and there is an assertion to check this in
> >virtio_add_queue()).
> >
> >Guests are supposed to honor the vring queue size, although indirect
> >descriptors seem to be able to use up to VIRTQUEUE_MAX_SIZE according to
> >my understanding of QEMU's virtio.c code.
> >
> >Why would you like to use IOV_MAX?
> 
> The idea was that IOV_MAX is the limit in case of at least virtio-blk. The 
> host
> will not support more than IOV_MAX iovecs passed to a block request.

Is there an issue in practice?

Stefan

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