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

Rusty Russell
2014-09-05T02:55:40Z

Andy Lutomirski <luto@amacapital.net> writes:
> On Tue, Sep 2, 2014 at 9:29 PM, Rusty Russell <rusty@rustcorp.com.au> 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)?

Agreed, it's a little hacky.

Here's the diff (I actually merged this into the patch, but no point
re-xmitting):

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a4ebbffac141..6d2b5310c991 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -131,7 +131,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
struct vring_virtqueue *vq = to_vvq(_vq);
struct scatterlist *sg;
struct vring_desc *desc;
- unsigned int i, n, avail, uninitialized_var(prev);
+ unsigned int i, n, avail, descs_used, uninitialized_var(prev);
int head;
bool indirect;

@@ -179,17 +179,18 @@ static inline int virtqueue_add(struct virtqueue *_vq,

/* Set up rest to use this indirect table. */
i = 0;
- total_sg = 1;
+ descs_used = 1;
indirect = true;
} else {
desc = vq->vring.desc;
i = head;
+ descs_used = total_sg;
indirect = false;
}

- if (vq->vq.num_free < total_sg) {
+ if (vq->vq.num_free < descs_used) {
pr_debug("Can't add buf len %i - avail = %i\n",
- total_sg, vq->vq.num_free);
+ descs_used, vq->vq.num_free);
/* FIXME: for historical reasons, we force a notify here if
* there are outgoing parts to the buffer. Presumably the
* host should service the ring ASAP. */
@@ -200,7 +201,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
}

/* We're about to use some buffers from the free list. */
- vq->vq.num_free -= total_sg;
+ vq->vq.num_free -= descs_used;

for (n = 0; n < out_sgs; n++) {
for (sg = sgs[n]; sg; sg = sg_next(sg)) {