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SubjectRe: [PATCH RFC v1 2/2] vhost: batching fetches
On Sat, Oct 12, 2019 at 03:30:52PM +0800, Jason Wang wrote:
>
> On 2019/10/11 下午9:46, Michael S. Tsirkin wrote:
> > 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>
> > ---
> > drivers/vhost/test.c | 2 +-
> > drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++-------
> > drivers/vhost/vhost.h | 4 +++-
> > 3 files changed, 46 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > index 39a018a7af2d..e3a8e9db22cd 100644
> > --- a/drivers/vhost/test.c
> > +++ b/drivers/vhost/test.c
> > @@ -128,7 +128,7 @@ 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);
> > f->private_data = n;
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 36661d6cb51f..aa383e847865 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -302,6 +302,7 @@ 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;
> > @@ -390,6 +391,7 @@ 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->batch_descs = dev->iov_limit - UIO_MAXIOV;
> > vq->descs = kmalloc_array(vq->max_descs,
> > sizeof(*vq->descs),
> > GFP_KERNEL);
> > @@ -2366,6 +2368,8 @@ static void pop_split_desc(struct vhost_virtqueue *vq)
> > --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;
> > @@ -2375,7 +2379,7 @@ static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc,
> > 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);
> > + h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
> > h->id = id;
> > return 0;
> > @@ -2450,7 +2454,7 @@ 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)
> > {
> > struct vring_desc desc;
> > unsigned int i, head, found = 0;
> > @@ -2462,7 +2466,11 @@ 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);
> > @@ -2541,6 +2549,24 @@ 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 <= vq->batch_descs)
> > + ret = fetch_buf(vq);
>
>
> It looks to me descriptor chaining might be broken here.

It should work because fetch_buf fetches a whole buf, following
the chain. Seems to work in a small test ... what issues do you see?

>
> > +
> > + 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
> > @@ -2562,6 +2588,8 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> > if (ret)
> > return ret;
> > + /* Note: indirect descriptors are not batched */
> > + /* TODO: batch up to a limit */
> > last = peek_split_desc(vq);
> > id = last->id;
> > @@ -2584,12 +2612,12 @@ int vhost_get_vq_desc_batch(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;
> > - if (desc->flags & ~(VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE)) {
> > + 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;
> > @@ -2628,15 +2656,21 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> > }
> > *out_num += ret;
> > }
> > +
> > + ret = desc->id;
> > +
> > + if (!(desc->flags & VRING_DESC_F_NEXT))
> > + break;
> > }
>
>
> What happens if we reach vq->ndescs but VRING_DESC_F_NEXT is still set?
>
> Thanks

This can't happen: descriptors are pushed by push_split_desc each time
we go through a loop in fetch_buf. The only way to exit the loop
with return code 0 is if next_desc return -1 that is when VRING_DESC_F_NEXT
is clear.

But it's a good idea to add a BUG_ON here, I'll do it in the next version.


>
> > - ret = id;
> > - 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 (!(desc->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 1724f61b6c2d..8b88e0c903da 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -100,7 +100,9 @@ struct vhost_virtqueue {
> > struct vhost_desc *descs;
> > int ndescs;
> > + int first_desc;
> > int max_descs;
> > + int batch_descs;
> > const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
> > struct file *kick;
> > @@ -245,7 +247,7 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
> > int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
> > #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)

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