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Subject[PATCH v8 19/19] vhost: batching fetches
With this patch applied, new and old code perform identically.

Lots of extra optimizations are now possible, e.g.
we can fetch multiple heads with copy_from/to_user now.
We can get rid of maintaining the log array. Etc etc.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200401183118.8334-4-eperezma@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/test.c | 2 +-
drivers/vhost/vhost.c | 47 ++++++++++++++++++++++++++++++++++++++-----
drivers/vhost/vhost.h | 5 ++++-
3 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
index b06680833f03..251ca723ac3f 100644
--- a/drivers/vhost/test.c
+++ b/drivers/vhost/test.c
@@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
dev = &n->dev;
vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
- vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
+ vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, NULL);

f->private_data = n;
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 6ca658c21e15..0395229486a9 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -299,6 +299,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
{
vq->num = 1;
vq->ndescs = 0;
+ vq->first_desc = 0;
vq->desc = NULL;
vq->avail = NULL;
vq->used = NULL;
@@ -367,6 +368,11 @@ static int vhost_worker(void *data)
return 0;
}

+static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
+{
+ return vq->max_descs - UIO_MAXIOV;
+}
+
static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
{
kfree(vq->descs);
@@ -389,6 +395,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
for (i = 0; i < dev->nvqs; ++i) {
vq = dev->vqs[i];
vq->max_descs = dev->iov_limit;
+ if (vhost_vq_num_batch_descs(vq) < 0) {
+ return -EINVAL;
+ }
vq->descs = kmalloc_array(vq->max_descs,
sizeof(*vq->descs),
GFP_KERNEL);
@@ -1570,6 +1579,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
vq->last_avail_idx = s.num;
/* Forget the cached index value. */
vq->avail_idx = vq->last_avail_idx;
+ vq->ndescs = vq->first_desc = 0;
break;
case VHOST_GET_VRING_BASE:
s.index = idx;
@@ -2136,7 +2146,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
return 0;
}

-static int fetch_descs(struct vhost_virtqueue *vq)
+static int fetch_buf(struct vhost_virtqueue *vq)
{
unsigned int i, head, found = 0;
struct vhost_desc *last;
@@ -2149,7 +2159,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
/* Check it isn't doing very strange things with descriptor numbers. */
last_avail_idx = vq->last_avail_idx;

- if (vq->avail_idx == vq->last_avail_idx) {
+ if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
+ /* If we already have work to do, don't bother re-checking. */
+ if (likely(vq->ndescs))
+ return vq->num;
+
if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
vq_err(vq, "Failed to access avail idx at %p\n",
&vq->avail->idx);
@@ -2240,6 +2254,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
return 0;
}

+static int fetch_descs(struct vhost_virtqueue *vq)
+{
+ int ret = 0;
+
+ if (unlikely(vq->first_desc >= vq->ndescs)) {
+ vq->first_desc = 0;
+ vq->ndescs = 0;
+ }
+
+ if (vq->ndescs)
+ return 0;
+
+ while (!ret && vq->ndescs <= vhost_vq_num_batch_descs(vq))
+ ret = fetch_buf(vq);
+
+ return vq->ndescs ? 0 : ret;
+}
+
/* This looks in the virtqueue and for the first available buffer, and converts
* it to an iovec for convenient access. Since descriptors consist of some
* number of output then some number of input descriptors, it's actually two
@@ -2265,7 +2297,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
if (unlikely(log))
*log_num = 0;

- for (i = 0; i < vq->ndescs; ++i) {
+ for (i = vq->first_desc; i < vq->ndescs; ++i) {
unsigned iov_count = *in_num + *out_num;
struct vhost_desc *desc = &vq->descs[i];
int access;
@@ -2311,14 +2343,19 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
}

ret = desc->id;
+
+ if (!(desc->flags & VRING_DESC_F_NEXT))
+ break;
}

- vq->ndescs = 0;
+ vq->first_desc = i + 1;

return ret;

err:
- vhost_discard_vq_desc(vq, 1);
+ for (i = vq->first_desc; i < vq->ndescs; ++i)
+ if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
+ vhost_discard_vq_desc(vq, 1);
vq->ndescs = 0;

return ret;
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 76356edee8e5..a67bda9792ec 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -81,6 +81,7 @@ struct vhost_virtqueue {

struct vhost_desc *descs;
int ndescs;
+ int first_desc;
int max_descs;

struct file *kick;
@@ -229,7 +230,7 @@ void vhost_iotlb_map_free(struct vhost_iotlb *iotlb,
struct vhost_iotlb_map *map);

#define vq_err(vq, fmt, ...) do { \
- pr_debug(pr_fmt(fmt), ##__VA_ARGS__); \
+ pr_err(pr_fmt(fmt), ##__VA_ARGS__); \
if ((vq)->error_ctx) \
eventfd_signal((vq)->error_ctx, 1);\
} while (0)
@@ -255,6 +256,8 @@ static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
void *private_data)
{
vq->private_data = private_data;
+ vq->ndescs = 0;
+ vq->first_desc = 0;
}

/**
--
MST
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