Re: [Qemu-devel] value of VIRTQUEUE_MAX_SIZE

[Peter Lieven]

Am 06.02.2015 um 11:42 schrieb 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?

If there is no platform where IOV_MAX is less than 1024 then not.

Peter