On Mon, 20 Jun 2011 16:16:24 +0300, Gleb Natapov <[email protected]> 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. _______________________________________________ Virtualization mailing list [email protected] https://lists.linux-foundation.org/mailman/listinfo/virtualization
