virtio_ring: unify direct/indirect code paths.

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.

This slows down pktgen by less than 1/2 a percent (which uses direct
descriptors), as well as vring_bench, but it's far neater.

vring_bench before:
	1061485790-1104800648(1.08254e+09+/-6.6e+06)ns
vring_bench after:
	1125610268-1183528965(1.14172e+09+/-8e+06)ns

pktgen before:
   787781-796334(793165+/-2.4e+03)pps 365-369(367.5+/-1.2)Mb/sec (365530384-369498976(3.68028e+08+/-1.1e+06)bps) errors: 0

pktgen after:
   779988-790404(786391+/-2.5e+03)pps 361-366(364.35+/-1.3)Mb/sec (361914432-366747456(3.64885e+08+/-1.2e+06)bps) errors: 0

Now, if we make force indirect descriptors by turning off any_header_sg
in virtio_net.c:

pktgen before:
  713773-721062(718374+/-2.1e+03)pps 331-334(332.95+/-0.92)Mb/sec (331190672-334572768(3.33325e+08+/-9.6e+05)bps) errors: 0
pktgen after:
  710542-719195(714898+/-2.4e+03)pps 329-333(331.15+/-1.1)Mb/sec (329691488-333706480(3.31713e+08+/-1.1e+06)bps) errors: 0

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 10a7c02..3b1f89b 100644 (file)
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -99,18 +99,10 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
-/* Set up an indirect table of descriptors and add it to the queue. */
-static inline int vring_add_indirect(struct vring_virtqueue *vq,
-                                    struct scatterlist *sgs[],
-                                    unsigned int total_sg,
-                                    unsigned int out_sgs,
-                                    unsigned int in_sgs,
-                                    gfp_t gfp)
+static struct vring_desc *alloc_indirect(unsigned int total_sg, gfp_t gfp)
 {
        struct vring_desc *desc;
-       unsigned head;
-       struct scatterlist *sg;
-       int i, n;
+       unsigned int i;
 
        /*
         * We require lowmem mappings for the descriptors because
@@ -121,49 +113,11 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
 
        desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
        if (!desc)
-               return -ENOMEM;
-
-       /* Transfer entries from the sg lists into the indirect page */
-       i = 0;
-       for (n = 0; n < out_sgs; n++) {
-               for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-                       desc[i].flags = VRING_DESC_F_NEXT;
-                       desc[i].addr = sg_phys(sg);
-                       desc[i].len = sg->length;
-                       desc[i].next = i+1;
-                       i++;
-               }
-       }
-       for (; n < (out_sgs + in_sgs); n++) {
-               for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-                       desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-                       desc[i].addr = sg_phys(sg);
-                       desc[i].len = sg->length;
-                       desc[i].next = i+1;
-                       i++;
-               }
-       }
-       BUG_ON(i != total_sg);
-
-       /* Last one doesn't continue. */
-       desc[i-1].flags &= ~VRING_DESC_F_NEXT;
-       desc[i-1].next = 0;
-
-       /* We're about to use a buffer */
-       vq->vq.num_free--;
-
-       /* Use a single buffer which doesn't continue */
-       head = vq->free_head;
-       vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
-       vq->vring.desc[head].addr = virt_to_phys(desc);
-       /* kmemleak gives a false positive, as it's hidden by virt_to_phys */
-       kmemleak_ignore(desc);
-       vq->vring.desc[head].len = i * sizeof(struct vring_desc);
-
-       /* Update free pointer */
-       vq->free_head = vq->vring.desc[head].next;
+               return NULL;
 
-       return head;
+       for (i = 0; i < total_sg; i++)
+               desc[i].next = i+1;
+       return desc;
 }
 
 static inline int virtqueue_add(struct virtqueue *_vq,
@@ -176,8 +130,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 {
        struct vring_virtqueue *vq = to_vvq(_vq);
        struct scatterlist *sg;
-       unsigned int i, n, avail, uninitialized_var(prev);
+       struct vring_desc *desc;
+       unsigned int i, n, avail, descs_used, uninitialized_var(prev);
        int head;
+       bool indirect;
 
        START_USE(vq);
 
@@ -201,21 +157,40 @@ static inline int virtqueue_add(struct virtqueue *_vq,
        }
 #endif
 
+       BUG_ON(total_sg > vq->vring.num);
+       BUG_ON(total_sg == 0);
+
+       head = vq->free_head;
+
        /* If the host supports indirect descriptor tables, and we have multiple
         * buffers, then go indirect. FIXME: tune this threshold */
-       if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
-               head = vring_add_indirect(vq, sgs, total_sg,
-                                         out_sgs, in_sgs, gfp);
-               if (likely(head >= 0))
-                       goto add_head;
+       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;
+               descs_used = 1;
+               indirect = true;
+       } else {
+               desc = vq->vring.desc;
+               i = head;
+               descs_used = total_sg;
+               indirect = false;
        }
 
-       BUG_ON(total_sg > vq->vring.num);
-       BUG_ON(total_sg == 0);
-
-       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. */
@@ -226,34 +201,35 @@ 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;
 
-       head = i = vq->free_head;
        for (n = 0; n < out_sgs; n++) {
                for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-                       vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
-                       vq->vring.desc[i].addr = sg_phys(sg);
-                       vq->vring.desc[i].len = sg->length;
+                       desc[i].flags = VRING_DESC_F_NEXT;
+                       desc[i].addr = sg_phys(sg);
+                       desc[i].len = sg->length;
                        prev = i;
-                       i = vq->vring.desc[i].next;
+                       i = desc[i].next;
                }
        }
        for (; n < (out_sgs + in_sgs); n++) {
                for (sg = sgs[n]; sg; sg = sg_next(sg)) {
-                       vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
-                       vq->vring.desc[i].addr = sg_phys(sg);
-                       vq->vring.desc[i].len = sg->length;
+                       desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
+                       desc[i].addr = sg_phys(sg);
+                       desc[i].len = sg->length;
                        prev = i;
-                       i = vq->vring.desc[i].next;
+                       i = desc[i].next;
                }
        }
        /* Last one doesn't continue. */
-       vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
+       desc[prev].flags &= ~VRING_DESC_F_NEXT;
 
        /* Update free pointer */
-       vq->free_head = i;
+       if (indirect)
+               vq->free_head = vq->vring.desc[head].next;
+       else
+               vq->free_head = i;
 
-add_head:
        /* Set token. */
        vq->data[head] = data;