Re: [PATCH 3/3] virtio_ring: unify direct/indirect code paths.

On Tue, Sep 2, 2014 at 9:29 PM, Rusty Russell [off-list ref] wrote:

virtqueue_add() populates the virtqueue descriptor table from the sgs
given.  If it uses an indirect descriptor table, then it puts a single
descriptor in the descriptor table pointing to the kmalloc'ed indirect
table where the sg is populated.

Previously vring_add_indirect() did the allocation and the simple
linear layout.  We replace that with alloc_indirect() which allocates
the indirect table then chains it like the normal descriptor table so
we can reuse the core logic.

+       if (vq->indirect && total_sg > 1 && vq->vq.num_free)
+               desc = alloc_indirect(total_sg, gfp);
+       else
+               desc = NULL;
+
+       if (desc) {
+               /* Use a single buffer which doesn't continue */
+               vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
+               vq->vring.desc[head].addr = virt_to_phys(desc);
+               /* avoid kmemleak false positive (hidden by virt_to_phys) */
+               kmemleak_ignore(desc);
+               vq->vring.desc[head].len = total_sg * sizeof(struct vring_desc);
+
+               /* Set up rest to use this indirect table. */
+               i = 0;
+               total_sg = 1;

This is a little too magical for me.  Would it make sense to add a new
variable for this (total_root_descs or something)?

--Andy