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Message-ID: <3be186d8-cde6-aa08-0446-1ec7fec0efe0@redhat.com>
Date: Sat, 12 Oct 2019 15:30:52 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc: kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org
Subject: Re: [PATCH RFC v1 2/2] vhost: batching fetches
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@...hat.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.
> +
> + 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
>
> - 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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