virtio_ring: assume sgs are always well-formed.

We used to have several callers which just used arrays.  They're
gone, so we can use sg_next() everywhere, simplifying the code.

On my laptop, this slowed down vring_bench by 15%:

vring_bench before:
	936153354-967745359(9.44739e+08+/-6.1e+06)ns
vring_bench after:
	1061485790-1104800648(1.08254e+09+/-6.6e+06)ns

However, a more realistic test using pktgen on a AMD FX(tm)-8320 saw
a few percent improvement:

pktgen before:
  767390-792966(785159+/-6.5e+03)pps 356-367(363.75+/-2.9)Mb/sec (356068960-367936224(3.64314e+08+/-3e+06)bps) errors: 0

pktgen after:
   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

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

1 files changed, 19 insertions, 49 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 4d08f45a9c29..10a7c0205440 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -99,28 +99,10 @@ struct vring_virtqueue
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
-static inline struct scatterlist *sg_next_chained(struct scatterlist *sg,
-                                                  unsigned int *count)
-{
-        return sg_next(sg);
-}
-
-static inline struct scatterlist *sg_next_arr(struct scatterlist *sg,
-                                              unsigned int *count)
-{
-        if (--(*count) == 0)
-                return NULL;
-        return sg + 1;
-}
-
 /* 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[],
-                                     struct scatterlist *(*next)
-                                       (struct scatterlist *, unsigned int *),
                                      unsigned int total_sg,
-                                     unsigned int total_out,
-                                     unsigned int total_in,
                                      unsigned int out_sgs,
                                      unsigned int in_sgs,
                                      gfp_t gfp)
@@ -144,7 +126,7 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
         /* 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 = next(sg, &total_out)) {
+                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;
@@ -153,7 +135,7 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
                 }
         }
         for (; n < (out_sgs + in_sgs); n++) {
-                for (sg = sgs[n]; sg; sg = next(sg, &total_in)) {
+                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;
@@ -186,10 +168,7 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
 
 static inline int virtqueue_add(struct virtqueue *_vq,
                                 struct scatterlist *sgs[],
-                                struct scatterlist *(*next)
-                                  (struct scatterlist *, unsigned int *),
-                                unsigned int total_out,
-                                unsigned int total_in,
+                                unsigned int total_sg,
                                 unsigned int out_sgs,
                                 unsigned int in_sgs,
                                 void *data,
@@ -197,7 +176,7 @@ 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), total_sg;
+        unsigned int i, n, avail, uninitialized_var(prev);
         int head;
 
         START_USE(vq);
@@ -222,13 +201,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
         }
 #endif
 
-        total_sg = total_in + total_out;
-
         /* 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, next, total_sg, total_out,
-                                          total_in,
+                head = vring_add_indirect(vq, sgs, total_sg,
                                           out_sgs, in_sgs, gfp);
                 if (likely(head >= 0))
                         goto add_head;
@@ -254,7 +230,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 
         head = i = vq->free_head;
         for (n = 0; n < out_sgs; n++) {
-                for (sg = sgs[n]; sg; sg = next(sg, &total_out)) {
+                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;
@@ -263,7 +239,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
                 }
         }
         for (; n < (out_sgs + in_sgs); n++) {
-                for (sg = sgs[n]; sg; sg = next(sg, &total_in)) {
+                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;
@@ -324,29 +300,23 @@ int virtqueue_add_sgs(struct virtqueue *_vq,
                       void *data,
                       gfp_t gfp)
 {
-        unsigned int i, total_out, total_in;
+        unsigned int i, total_sg = 0;
 
         /* Count them first. */
-        for (i = total_out = total_in = 0; i < out_sgs; i++) {
-                struct scatterlist *sg;
-                for (sg = sgs[i]; sg; sg = sg_next(sg))
-                        total_out++;
-        }
-        for (; i < out_sgs + in_sgs; i++) {
+        for (i = 0; i < out_sgs + in_sgs; i++) {
                 struct scatterlist *sg;
                 for (sg = sgs[i]; sg; sg = sg_next(sg))
-                        total_in++;
+                        total_sg++;
         }
-        return virtqueue_add(_vq, sgs, sg_next_chained,
-                             total_out, total_in, out_sgs, in_sgs, data, gfp);
+        return virtqueue_add(_vq, sgs, total_sg, out_sgs, in_sgs, data, gfp);
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
 
 /**
  * virtqueue_add_outbuf - expose output buffers to other end
  * @vq: the struct virtqueue we're talking about.
- * @sgs: array of scatterlists (need not be terminated!)
- * @num: the number of scatterlists readable by other side
+ * @sg: scatterlist (must be well-formed and terminated!)
+ * @num: the number of entries in @sg readable by other side
  * @data: the token identifying the buffer.
  * @gfp: how to do memory allocations (if necessary).
  *
@@ -356,19 +326,19 @@ EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
  * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
  */
 int virtqueue_add_outbuf(struct virtqueue *vq,
-                         struct scatterlist sg[], unsigned int num,
+                         struct scatterlist *sg, unsigned int num,
                          void *data,
                          gfp_t gfp)
 {
-        return virtqueue_add(vq, &sg, sg_next_arr, num, 0, 1, 0, data, gfp);
+        return virtqueue_add(vq, &sg, num, 1, 0, data, gfp);
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
 
 /**
  * virtqueue_add_inbuf - expose input buffers to other end
  * @vq: the struct virtqueue we're talking about.
- * @sgs: array of scatterlists (need not be terminated!)
- * @num: the number of scatterlists writable by other side
+ * @sg: scatterlist (must be well-formed and terminated!)
+ * @num: the number of entries in @sg writable by other side
  * @data: the token identifying the buffer.
  * @gfp: how to do memory allocations (if necessary).
  *
@@ -378,11 +348,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
  * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
  */
 int virtqueue_add_inbuf(struct virtqueue *vq,
-                        struct scatterlist sg[], unsigned int num,
+                        struct scatterlist *sg, unsigned int num,
                         void *data,
                         gfp_t gfp)
 {
-        return virtqueue_add(vq, &sg, sg_next_arr, 0, num, 0, 1, data, gfp);
+        return virtqueue_add(vq, &sg, num, 0, 1, data, gfp);
 }
 EXPORT_SYMBOL_GPL(virtqueue_add_inbuf);