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Date: Thu, 05 Feb 2015 15:29:17 +0100
From: Peter Lieven <pl@kamp.de>
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Subject: Re: [Qemu-devel] value of VIRTQUEUE_MAX_SIZE
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, ming.lei@canonical.com, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

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.

Peter