Re: [Qemu-devel] [PATCH 0/6] virtio: handle non contigious s/g entries

[Stefan Hajnoczi]

On Tue, Oct 27, 2015 at 01:51:20PM +0200, Michael S. Tsirkin wrote:
> On Tue, Oct 27, 2015 at 10:47:54AM +0200, Michael S. Tsirkin wrote:
> > TL;DR:
> > This fixes virtio in a way transparent to guest.
> > We should now be able to revert commits aa8580cd and df0acded19ec which 
> > worked
> > around it in a way that's not transparent.
> 
> I didn't check dataplane BTW. Igor? Stefan?

It doesn't handle it:

    /* TODO handle non-contiguous memory across region boundaries */
    iov->iov_base = vring_map(&mr, desc->addr, desc->len,
                              desc->flags & VRING_DESC_F_WRITE);
    if (!iov->iov_base) {
        error_report("Failed to map descriptor addr %#" PRIx64 " len %u",
                     (uint64_t)desc->addr, desc->len);
        return -EFAULT;
    }

The assumption is that vring_map() can map the full desc->len bytes.

The same logic that you're trying to add in this series could be added to
vring_pop() but it makes the number of sg list entries accepted by QEMU
variable and dependent on DIMMs :(.  Previously guests could be sure that 1024
sgs work.