Re: [Qemu-devel] value of VIRTQUEUE_MAX_SIZE

[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?

Stefan

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