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virtio_ring: unify direct/indirect code paths.
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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>
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Rusty Russell authored and David S. Miller committed Sep 13, 2014
1 parent eeebf9b commit b25bd25
Showing 1 changed file with 52 additions and 76 deletions.
128 changes: 52 additions & 76 deletions drivers/virtio/virtio_ring.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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,
Expand All @@ -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);

Expand All @@ -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. */
Expand All @@ -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;

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