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Subject[PATCH v8 17/19] vhost: option to fetch descriptors through an independent struct
The idea is to support multiple ring formats by converting
to a format-independent array of descriptors.

This costs extra cycles, but we gain in ability
to fetch a batch of descriptors in one go, which
is good for code cache locality.

When used, this causes a minor performance degradation,
it's been kept as simple as possible for ease of review.
A follow-up patch gets us back the performance by adding batching.

To simplify benchmarking, I kept the old code around so one can switch
back and forth between old and new code. This will go away in the final
submission.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200401183118.8334-2-eperezma@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
drivers/vhost/vhost.c | 297 +++++++++++++++++++++++++++++++++++++++++-
drivers/vhost/vhost.h | 16 +++
2 files changed, 312 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d450e16c5c25..56593ba6decc 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -298,6 +298,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
struct vhost_virtqueue *vq)
{
vq->num = 1;
+ vq->ndescs = 0;
vq->desc = NULL;
vq->avail = NULL;
vq->used = NULL;
@@ -368,6 +369,9 @@ static int vhost_worker(void *data)

static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
{
+ kfree(vq->descs);
+ vq->descs = NULL;
+ vq->max_descs = 0;
kfree(vq->indirect);
vq->indirect = NULL;
kfree(vq->log);
@@ -384,6 +388,10 @@ 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;
+ vq->descs = kmalloc_array(vq->max_descs,
+ sizeof(*vq->descs),
+ GFP_KERNEL);
vq->indirect = kmalloc_array(UIO_MAXIOV,
sizeof(*vq->indirect),
GFP_KERNEL);
@@ -391,7 +399,7 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
GFP_KERNEL);
vq->heads = kmalloc_array(dev->iov_limit, sizeof(*vq->heads),
GFP_KERNEL);
- if (!vq->indirect || !vq->log || !vq->heads)
+ if (!vq->indirect || !vq->log || !vq->heads || !vq->descs)
goto err_nomem;
}
return 0;
@@ -2277,6 +2285,293 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
}
EXPORT_SYMBOL_GPL(vhost_get_vq_desc);

+static struct vhost_desc *peek_split_desc(struct vhost_virtqueue *vq)
+{
+ BUG_ON(!vq->ndescs);
+ return &vq->descs[vq->ndescs - 1];
+}
+
+static void pop_split_desc(struct vhost_virtqueue *vq)
+{
+ BUG_ON(!vq->ndescs);
+ --vq->ndescs;
+}
+
+#define VHOST_DESC_FLAGS (VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE | \
+ VRING_DESC_F_NEXT)
+static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc, u16 id)
+{
+ struct vhost_desc *h;
+
+ if (unlikely(vq->ndescs >= vq->max_descs))
+ return -EINVAL;
+ h = &vq->descs[vq->ndescs++];
+ h->addr = vhost64_to_cpu(vq, desc->addr);
+ h->len = vhost32_to_cpu(vq, desc->len);
+ h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
+ h->id = id;
+
+ return 0;
+}
+
+static int fetch_indirect_descs(struct vhost_virtqueue *vq,
+ struct vhost_desc *indirect,
+ u16 head)
+{
+ struct vring_desc desc;
+ unsigned int i = 0, count, found = 0;
+ u32 len = indirect->len;
+ struct iov_iter from;
+ int ret;
+
+ /* Sanity check */
+ if (unlikely(len % sizeof desc)) {
+ vq_err(vq, "Invalid length in indirect descriptor: "
+ "len 0x%llx not multiple of 0x%zx\n",
+ (unsigned long long)len,
+ sizeof desc);
+ return -EINVAL;
+ }
+
+ ret = translate_desc(vq, indirect->addr, len, vq->indirect,
+ UIO_MAXIOV, VHOST_ACCESS_RO);
+ if (unlikely(ret < 0)) {
+ if (ret != -EAGAIN)
+ vq_err(vq, "Translation failure %d in indirect.\n", ret);
+ return ret;
+ }
+ iov_iter_init(&from, READ, vq->indirect, ret, len);
+
+ /* We will use the result as an address to read from, so most
+ * architectures only need a compiler barrier here. */
+ read_barrier_depends();
+
+ count = len / sizeof desc;
+ /* Buffers are chained via a 16 bit next field, so
+ * we can have at most 2^16 of these. */
+ if (unlikely(count > USHRT_MAX + 1)) {
+ vq_err(vq, "Indirect buffer length too big: %d\n",
+ indirect->len);
+ return -E2BIG;
+ }
+ if (unlikely(vq->ndescs + count > vq->max_descs)) {
+ vq_err(vq, "Too many indirect + direct descs: %d + %d\n",
+ vq->ndescs, indirect->len);
+ return -E2BIG;
+ }
+
+ do {
+ if (unlikely(++found > count)) {
+ vq_err(vq, "Loop detected: last one at %u "
+ "indirect size %u\n",
+ i, count);
+ return -EINVAL;
+ }
+ if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) {
+ vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
+ i, (size_t)indirect->addr + i * sizeof desc);
+ return -EINVAL;
+ }
+ if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
+ vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
+ i, (size_t)indirect->addr + i * sizeof desc);
+ return -EINVAL;
+ }
+
+ push_split_desc(vq, &desc, head);
+ } while ((i = next_desc(vq, &desc)) != -1);
+ return 0;
+}
+
+static int fetch_descs(struct vhost_virtqueue *vq)
+{
+ unsigned int i, head, found = 0;
+ struct vhost_desc *last;
+ struct vring_desc desc;
+ __virtio16 avail_idx;
+ __virtio16 ring_head;
+ u16 last_avail_idx;
+ int ret;
+
+ /* 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(vhost_get_avail_idx(vq, &avail_idx))) {
+ vq_err(vq, "Failed to access avail idx at %p\n",
+ &vq->avail->idx);
+ return -EFAULT;
+ }
+ vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+
+ if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
+ vq_err(vq, "Guest moved used index from %u to %u",
+ last_avail_idx, vq->avail_idx);
+ return -EFAULT;
+ }
+
+ /* If there's nothing new since last we looked, return
+ * invalid.
+ */
+ if (vq->avail_idx == last_avail_idx)
+ return vq->num;
+
+ /* Only get avail ring entries after they have been
+ * exposed by guest.
+ */
+ smp_rmb();
+ }
+
+ /* Grab the next descriptor number they're advertising */
+ if (unlikely(vhost_get_avail_head(vq, &ring_head, last_avail_idx))) {
+ vq_err(vq, "Failed to read head: idx %d address %p\n",
+ last_avail_idx,
+ &vq->avail->ring[last_avail_idx % vq->num]);
+ return -EFAULT;
+ }
+
+ head = vhost16_to_cpu(vq, ring_head);
+
+ /* If their number is silly, that's an error. */
+ if (unlikely(head >= vq->num)) {
+ vq_err(vq, "Guest says index %u > %u is available",
+ head, vq->num);
+ return -EINVAL;
+ }
+
+ i = head;
+ do {
+ if (unlikely(i >= vq->num)) {
+ vq_err(vq, "Desc index is %u > %u, head = %u",
+ i, vq->num, head);
+ return -EINVAL;
+ }
+ if (unlikely(++found > vq->num)) {
+ vq_err(vq, "Loop detected: last one at %u "
+ "vq size %u head %u\n",
+ i, vq->num, head);
+ return -EINVAL;
+ }
+ ret = vhost_get_desc(vq, &desc, i);
+ if (unlikely(ret)) {
+ vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
+ i, vq->desc + i);
+ return -EFAULT;
+ }
+ ret = push_split_desc(vq, &desc, head);
+ if (unlikely(ret)) {
+ vq_err(vq, "Failed to save descriptor: idx %d\n", i);
+ return -EINVAL;
+ }
+ } while ((i = next_desc(vq, &desc)) != -1);
+
+ last = peek_split_desc(vq);
+ if (unlikely(last->flags & VRING_DESC_F_INDIRECT)) {
+ pop_split_desc(vq);
+ ret = fetch_indirect_descs(vq, last, head);
+ if (unlikely(ret < 0)) {
+ if (ret != -EAGAIN)
+ vq_err(vq, "Failure detected "
+ "in indirect descriptor at idx %d\n", head);
+ return ret;
+ }
+ }
+
+ /* Assume notifications from guest are disabled at this point,
+ * if they aren't we would need to update avail_event index. */
+ BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
+
+ /* On success, increment avail index. */
+ vq->last_avail_idx++;
+
+ return 0;
+}
+
+/* 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
+ * iovecs, but we pack them into one and note how many of each there were.
+ *
+ * This function returns the descriptor number found, or vq->num (which is
+ * never a valid descriptor number) if none was found. A negative code is
+ * returned on error. */
+int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
+ struct iovec iov[], unsigned int iov_size,
+ unsigned int *out_num, unsigned int *in_num,
+ struct vhost_log *log, unsigned int *log_num)
+{
+ int ret = fetch_descs(vq);
+ int i;
+
+ if (ret)
+ return ret;
+
+ /* Now convert to IOV */
+ /* When we start there are none of either input nor output. */
+ *out_num = *in_num = 0;
+ if (unlikely(log))
+ *log_num = 0;
+
+ for (i = 0; i < vq->ndescs; ++i) {
+ unsigned iov_count = *in_num + *out_num;
+ struct vhost_desc *desc = &vq->descs[i];
+ int access;
+
+ if (desc->flags & ~VHOST_DESC_FLAGS) {
+ vq_err(vq, "Unexpected flags: 0x%x at descriptor id 0x%x\n",
+ desc->flags, desc->id);
+ ret = -EINVAL;
+ goto err;
+ }
+ if (desc->flags & VRING_DESC_F_WRITE)
+ access = VHOST_ACCESS_WO;
+ else
+ access = VHOST_ACCESS_RO;
+ ret = translate_desc(vq, desc->addr,
+ desc->len, iov + iov_count,
+ iov_size - iov_count, access);
+ if (unlikely(ret < 0)) {
+ if (ret != -EAGAIN)
+ vq_err(vq, "Translation failure %d descriptor idx %d\n",
+ ret, i);
+ goto err;
+ }
+ if (access == VHOST_ACCESS_WO) {
+ /* If this is an input descriptor,
+ * increment that count. */
+ *in_num += ret;
+ if (unlikely(log && ret)) {
+ log[*log_num].addr = desc->addr;
+ log[*log_num].len = desc->len;
+ ++*log_num;
+ }
+ } else {
+ /* If it's an output descriptor, they're all supposed
+ * to come before any input descriptors. */
+ if (unlikely(*in_num)) {
+ vq_err(vq, "Descriptor has out after in: "
+ "idx %d\n", i);
+ ret = -EINVAL;
+ goto err;
+ }
+ *out_num += ret;
+ }
+
+ ret = desc->id;
+ }
+
+ vq->ndescs = 0;
+
+ return ret;
+
+err:
+ vhost_discard_vq_desc(vq, 1);
+ vq->ndescs = 0;
+
+ return ret;
+}
+EXPORT_SYMBOL_GPL(vhost_get_vq_desc_batch);
+
/* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
{
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 60cab4c78229..0976a2853935 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -60,6 +60,13 @@ enum vhost_uaddr_type {
VHOST_NUM_ADDRS = 3,
};

+struct vhost_desc {
+ u64 addr;
+ u32 len;
+ u16 flags; /* VRING_DESC_F_WRITE, VRING_DESC_F_NEXT */
+ u16 id;
+};
+
/* The virtqueue structure describes a queue attached to a device. */
struct vhost_virtqueue {
struct vhost_dev *dev;
@@ -71,6 +78,11 @@ struct vhost_virtqueue {
vring_avail_t __user *avail;
vring_used_t __user *used;
const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS];
+
+ struct vhost_desc *descs;
+ int ndescs;
+ int max_descs;
+
struct file *kick;
struct eventfd_ctx *call_ctx;
struct eventfd_ctx *error_ctx;
@@ -175,6 +187,10 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
bool vhost_vq_access_ok(struct vhost_virtqueue *vq);
bool vhost_log_access_ok(struct vhost_dev *);

+int vhost_get_vq_desc_batch(struct vhost_virtqueue *,
+ struct iovec iov[], unsigned int iov_count,
+ unsigned int *out_num, unsigned int *in_num,
+ struct vhost_log *log, unsigned int *log_num);
int vhost_get_vq_desc(struct vhost_virtqueue *,
struct iovec iov[], unsigned int iov_count,
unsigned int *out_num, unsigned int *in_num,
--
MST
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