Re: Make virtio vq size configurable by a guest.

[Rusty Russell <rusty@rustcorp.com.au>]

On Mon, 20 Jun 2011 16:16:24 +0300, Gleb Natapov <gleb@redhat.com> wrote:
> Hi,
> 
> Currently in virtio host dictates the size and layout of vq that should
> be used.  To talk to a device that has one vq with 128 elements guest
> needs to allocate at least 2 pages. Usually this is not a problem, but
> sometimes guest runs in a resource restricted environment and then it may
> not have enough memory to initialize all virtio devices present in the
> system. One such environment is a BIOS. Seabios currently has virtio block
> support. Since the BIOS should be able to access the disk even after OS
> is launched vq should be allocated from a special memory region that will
> be marked as unavailable to an OS, but such memory is scarce. Because vq
> is so huge only a couple of virtio disks can be initialized by the BIOS.
> 
> It would be nice if a guest will be able to tell to a host what vq size
> should be used instead. BIOS issues only one request at a time anyway,
> so it needs only one element in vq. It does not care about performance
> to much either, so it can tell to a host to not align used index to a
> page boundary. This way vq of one element shouldn't take more then a couple
> hundreds of bytes.

Unfortunately, a virtqueue *always* takes at least 2 pages.  That's
because we split the host/guest part on page boundaries.  (2 pages per
disk is "huge"?  Really?)

So really, you want to negotiate the ring size and the 'align'
parameter.  A new feature could allow this, but there may be valid
reasons for a host to want to place an upper limit, too.

Thanks,
Rusty.