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Message-ID: <f9cc1fe1-79cd-40ec-8fa9-88a14f1b2ca1@oracle.com>
Date: Wed, 9 Jul 2025 12:38:23 -0400
From: Jonah Palmer <jonah.palmer@...cle.com>
To: Jason Wang <jasowang@...hat.com>, mst@...hat.com, eperezma@...hat.com
Cc: kvm@...r.kernel.org, virtualization@...ts.linux.dev,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] vhost: basic in order support
On 7/8/25 2:48 AM, Jason Wang wrote:
> This patch adds basic in order support for vhost. Two optimizations
> are implemented in this patch:
>
> 1) Since driver uses descriptor in order, vhost can deduce the next
> avail ring head by counting the number of descriptors that has been
> used in next_avail_head. This eliminate the need to access the
> available ring in vhost.
>
> 2) vhost_add_used_and_singal_n() is extended to accept the number of
> batched buffers per used elem. While this increases the times of
> usersapce memory access but it helps to reduce the chance of
> used ring access of both the driver and vhost.
>
> Vhost-net will be the first user for this.
Acked-by: Jonah Palmer <jonah.palmer@...cle.com>
>
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
> drivers/vhost/net.c | 6 ++-
> drivers/vhost/vhost.c | 121 +++++++++++++++++++++++++++++++++++-------
> drivers/vhost/vhost.h | 8 ++-
> 3 files changed, 111 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 7cbfc7d718b3..4f9c67f17b49 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -374,7 +374,8 @@ static void vhost_zerocopy_signal_used(struct vhost_net *net,
> while (j) {
> add = min(UIO_MAXIOV - nvq->done_idx, j);
> vhost_add_used_and_signal_n(vq->dev, vq,
> - &vq->heads[nvq->done_idx], add);
> + &vq->heads[nvq->done_idx],
> + NULL, add);
> nvq->done_idx = (nvq->done_idx + add) % UIO_MAXIOV;
> j -= add;
> }
> @@ -457,7 +458,8 @@ static void vhost_net_signal_used(struct vhost_net_virtqueue *nvq)
> if (!nvq->done_idx)
> return;
>
> - vhost_add_used_and_signal_n(dev, vq, vq->heads, nvq->done_idx);
> + vhost_add_used_and_signal_n(dev, vq, vq->heads, NULL,
> + nvq->done_idx);
> nvq->done_idx = 0;
> }
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 3a5ebb973dba..c7ed069fc49e 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -364,6 +364,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> vq->avail = NULL;
> vq->used = NULL;
> vq->last_avail_idx = 0;
> + vq->next_avail_head = 0;
> vq->avail_idx = 0;
> vq->last_used_idx = 0;
> vq->signalled_used = 0;
> @@ -455,6 +456,8 @@ static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> vq->log = NULL;
> kfree(vq->heads);
> vq->heads = NULL;
> + kfree(vq->nheads);
> + vq->nheads = NULL;
> }
>
> /* Helper to allocate iovec buffers for all vqs. */
> @@ -472,7 +475,9 @@ 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)
> + vq->nheads = kmalloc_array(dev->iov_limit, sizeof(*vq->nheads),
> + GFP_KERNEL);
> + if (!vq->indirect || !vq->log || !vq->heads || !vq->nheads)
> goto err_nomem;
> }
> return 0;
> @@ -1990,14 +1995,15 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> break;
> }
> if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
> - vq->last_avail_idx = s.num & 0xffff;
> + vq->next_avail_head = vq->last_avail_idx =
> + s.num & 0xffff;
> vq->last_used_idx = (s.num >> 16) & 0xffff;
> } else {
> if (s.num > 0xffff) {
> r = -EINVAL;
> break;
> }
> - vq->last_avail_idx = s.num;
> + vq->next_avail_head = vq->last_avail_idx = s.num;
> }
> /* Forget the cached index value. */
> vq->avail_idx = vq->last_avail_idx;
> @@ -2590,11 +2596,12 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> unsigned int *out_num, unsigned int *in_num,
> struct vhost_log *log, unsigned int *log_num)
> {
> + bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
> struct vring_desc desc;
> unsigned int i, head, found = 0;
> u16 last_avail_idx = vq->last_avail_idx;
> __virtio16 ring_head;
> - int ret, access;
> + int ret, access, c = 0;
>
> if (vq->avail_idx == vq->last_avail_idx) {
> ret = vhost_get_avail_idx(vq);
> @@ -2605,17 +2612,21 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> return vq->num;
> }
>
> - /* Grab the next descriptor number they're advertising, and increment
> - * the index we've seen. */
> - 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;
> + if (in_order)
> + head = vq->next_avail_head & (vq->num - 1);
> + else {
> + /* Grab the next descriptor number they're
> + * advertising, and increment the index we've seen. */
> + 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);
> }
>
> - 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",
> @@ -2658,6 +2669,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> "in indirect descriptor at idx %d\n", i);
> return ret;
> }
> + ++c;
> continue;
> }
>
> @@ -2693,10 +2705,12 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> }
> *out_num += ret;
> }
> + ++c;
> } while ((i = next_desc(vq, &desc)) != -1);
>
> /* On success, increment avail index. */
> vq->last_avail_idx++;
> + vq->next_avail_head += c;
>
> /* Assume notifications from guest are disabled at this point,
> * if they aren't we would need to update avail_event index. */
> @@ -2720,8 +2734,9 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
> cpu_to_vhost32(vq, head),
> cpu_to_vhost32(vq, len)
> };
> + u16 nheads = 1;
>
> - return vhost_add_used_n(vq, &heads, 1);
> + return vhost_add_used_n(vq, &heads, &nheads, 1);
> }
> EXPORT_SYMBOL_GPL(vhost_add_used);
>
> @@ -2757,10 +2772,10 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> return 0;
> }
>
> -/* After we've used one of their buffers, we tell them about it. We'll then
> - * want to notify the guest, using eventfd. */
> -int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> - unsigned count)
> +static int vhost_add_used_n_ooo(struct vhost_virtqueue *vq,
> + struct vring_used_elem *heads,
> + u16 *nheads,
> + unsigned count)
> {
> int start, n, r;
>
> @@ -2775,6 +2790,70 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> }
> r = __vhost_add_used_n(vq, heads, count);
>
> + return r;
> +}
> +
> +static int vhost_add_used_n_in_order(struct vhost_virtqueue *vq,
> + struct vring_used_elem *heads,
> + u16 *nheads,
> + unsigned count)
> +{
> + vring_used_elem_t __user *used;
> + u16 old, new = vq->last_used_idx;
> + int start, i;
> +
> + if (!nheads)
> + return -EINVAL;
> +
> + start = vq->last_used_idx & (vq->num - 1);
> + used = vq->used->ring + start;
> +
> + for (i = 0; i < count; i++) {
> + if (vhost_put_used(vq, &heads[i], start, 1)) {
> + vq_err(vq, "Failed to write used");
> + return -EFAULT;
> + }
> + start += nheads[i];
> + new += nheads[i];
> + if (start >= vq->num)
> + start -= vq->num;
> + }
> +
> + if (unlikely(vq->log_used)) {
> + /* Make sure data is seen before log. */
> + smp_wmb();
> + /* Log used ring entry write. */
> + log_used(vq, ((void __user *)used - (void __user *)vq->used),
> + (vq->num - start) * sizeof *used);
> + if (start + count > vq->num)
> + log_used(vq, 0,
> + (start + count - vq->num) * sizeof *used);
> + }
> +
> + old = vq->last_used_idx;
> + vq->last_used_idx = new;
> + /* If the driver never bothers to signal in a very long while,
> + * used index might wrap around. If that happens, invalidate
> + * signalled_used index we stored. TODO: make sure driver
> + * signals at least once in 2^16 and remove this. */
> + if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
> + vq->signalled_used_valid = false;
> + return 0;
> +}
> +
> +/* After we've used one of their buffers, we tell them about it. We'll then
> + * want to notify the guest, using eventfd. */
> +int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
> + u16 *nheads, unsigned count)
> +{
> + bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
> + int r;
> +
> + if (!in_order || !nheads)
> + r = vhost_add_used_n_ooo(vq, heads, nheads, count);
> + else
> + r = vhost_add_used_n_in_order(vq, heads, nheads, count);
> +
> /* Make sure buffer is written before we update index. */
> smp_wmb();
> if (vhost_put_used_idx(vq)) {
> @@ -2853,9 +2932,11 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal);
> /* multi-buffer version of vhost_add_used_and_signal */
> void vhost_add_used_and_signal_n(struct vhost_dev *dev,
> struct vhost_virtqueue *vq,
> - struct vring_used_elem *heads, unsigned count)
> + struct vring_used_elem *heads,
> + u16 *nheads,
> + unsigned count)
> {
> - vhost_add_used_n(vq, heads, count);
> + vhost_add_used_n(vq, heads, nheads, count);
> vhost_signal(dev, vq);
> }
> EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index bb75a292d50c..dca9f309d396 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -103,6 +103,8 @@ struct vhost_virtqueue {
> * Values are limited to 0x7fff, and the high bit is used as
> * a wrap counter when using VIRTIO_F_RING_PACKED. */
> u16 last_avail_idx;
> + /* Next avail ring head when VIRTIO_F_IN_ORDER is neogitated */
> + u16 next_avail_head;
>
> /* Caches available index value from user. */
> u16 avail_idx;
> @@ -129,6 +131,7 @@ struct vhost_virtqueue {
> struct iovec iotlb_iov[64];
> struct iovec *indirect;
> struct vring_used_elem *heads;
> + u16 *nheads;
> /* Protected by virtqueue mutex. */
> struct vhost_iotlb *umem;
> struct vhost_iotlb *iotlb;
> @@ -213,11 +216,12 @@ bool vhost_vq_is_setup(struct vhost_virtqueue *vq);
> int vhost_vq_init_access(struct vhost_virtqueue *);
> int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
> int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
> - unsigned count);
> + u16 *nheads, unsigned count);
> void vhost_add_used_and_signal(struct vhost_dev *, struct vhost_virtqueue *,
> unsigned int id, int len);
> void vhost_add_used_and_signal_n(struct vhost_dev *, struct vhost_virtqueue *,
> - struct vring_used_elem *heads, unsigned count);
> + struct vring_used_elem *heads, u16 *nheads,
> + unsigned count);
> void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
> void vhost_disable_notify(struct vhost_dev *, struct vhost_virtqueue *);
> bool vhost_vq_avail_empty(struct vhost_dev *, struct vhost_virtqueue *);
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