Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address

From: Michael S. Tsirkin
Date: Thu Dec 13 2018 - 10:45:01 EST

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In reply to: Jason Wang: "[PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address"
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On Thu, Dec 13, 2018 at 06:10:22PM +0800, Jason Wang wrote:
> It was noticed that the copy_user() friends that was used to access
> virtqueue metdata tends to be very expensive for dataplane
> implementation like vhost since it involves lots of software check,
> speculation barrier, hardware feature toggling (e.g SMAP). The
> extra cost will be more obvious when transferring small packets.
>
> This patch tries to eliminate those overhead by pin vq metadata pages
> and access them through vmap(). During SET_VRING_ADDR, we will setup
> those mappings and memory accessors are modified to use pointers to
> access the metadata directly.
>
> Note, this was only done when device IOTLB is not enabled. We could
> use similar method to optimize it in the future.
>
> Tests shows about ~24% improvement on TX PPS when using virtio-user +
> vhost_net + xdp1 on TAP (CONFIG_HARDENED_USERCOPY is not enabled):
>
> Before: ~5.0Mpps
> After: ~6.1Mpps
>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> drivers/vhost/vhost.c | 178 ++++++++++++++++++++++++++++++++++++++++++
> drivers/vhost/vhost.h | 11 +++
> 2 files changed, 189 insertions(+)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index bafe39d2e637..1bd24203afb6 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -443,6 +443,9 @@ void vhost_dev_init(struct vhost_dev *dev,
> vq->indirect = NULL;
> vq->heads = NULL;
> vq->dev = dev;
> + memset(&vq->avail_ring, 0, sizeof(vq->avail_ring));
> + memset(&vq->used_ring, 0, sizeof(vq->used_ring));
> + memset(&vq->desc_ring, 0, sizeof(vq->desc_ring));
> mutex_init(&vq->mutex);
> vhost_vq_reset(dev, vq);
> if (vq->handle_kick)
> @@ -614,6 +617,102 @@ static void vhost_clear_msg(struct vhost_dev *dev)
> spin_unlock(&dev->iotlb_lock);
> }
>
> +static int vhost_init_vmap(struct vhost_vmap *map, unsigned long uaddr,
> + size_t size, int write)
> +{
> + struct page **pages;
> + int npages = DIV_ROUND_UP(size, PAGE_SIZE);
> + int npinned;
> + void *vaddr;
> +
> + pages = kmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
> + if (!pages)
> + return -ENOMEM;
> +
> + npinned = get_user_pages_fast(uaddr, npages, write, pages);
> + if (npinned != npages)
> + goto err;
> +

As I said I have doubts about the whole approach, but this
implementation in particular isn't a good idea
as it keeps the page around forever.
So no THP, no NUMA rebalancing, userspace-controlled
amount of memory locked up and not accounted for.

Don't get me wrong it's a great patch in an ideal world.
But then in an ideal world no barriers smap etc are necessary at all.

> + vaddr = vmap(pages, npages, VM_MAP, PAGE_KERNEL);
> + if (!vaddr)
> + goto err;
> +
> + map->pages = pages;
> + map->addr = vaddr + (uaddr & (PAGE_SIZE - 1));
> + map->npages = npages;
> +
> + return 0;
> +
> +err:
> + if (npinned > 0)
> + release_pages(pages, npinned);
> + kfree(pages);
> + return -EFAULT;
> +}
> +
> +static void vhost_uninit_vmap(struct vhost_vmap *map)
> +{
> + if (!map->addr)
> + return;
> +
> + vunmap(map->addr);
> + release_pages(map->pages, map->npages);
> + kfree(map->pages);
> +
> + map->addr = NULL;
> + map->pages = NULL;
> + map->npages = 0;
> +}
> +
> +static void vhost_clean_vmaps(struct vhost_virtqueue *vq)
> +{
> + vhost_uninit_vmap(&vq->avail_ring);
> + vhost_uninit_vmap(&vq->desc_ring);
> + vhost_uninit_vmap(&vq->used_ring);
> +}
> +
> +static int vhost_setup_vmaps(struct vhost_virtqueue *vq, unsigned long avail,
> + unsigned long desc, unsigned long used)
> +{
> + size_t event = vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;
> + size_t avail_size, desc_size, used_size;
> + int ret;
> +
> + vhost_clean_vmaps(vq);
> +
> + avail_size = sizeof(*vq->avail) +
> + sizeof(*vq->avail->ring) * vq->num + event;
> + ret = vhost_init_vmap(&vq->avail_ring, avail, avail_size, false);
> + if (ret) {
> + vq_err(vq, "Fail to setup vmap for avail ring!\n");
> + goto err_avail;
> + }
> +
> + desc_size = sizeof(*vq->desc) * vq->num;
> + ret = vhost_init_vmap(&vq->desc_ring, desc, desc_size, false);
> + if (ret) {
> + vq_err(vq, "Fail to setup vmap for desc ring!\n");
> + goto err_desc;
> + }
> +
> + used_size = sizeof(*vq->used) +
> + sizeof(*vq->used->ring) * vq->num + event;
> + ret = vhost_init_vmap(&vq->used_ring, used, used_size, true);
> + if (ret) {
> + vq_err(vq, "Fail to setup vmap for used ring!\n");
> + goto err_used;
> + }
> +
> + return 0;
> +
> +err_used:
> + vhost_uninit_vmap(&vq->used_ring);
> +err_desc:
> + vhost_uninit_vmap(&vq->avail_ring);
> +err_avail:
> + return -EFAULT;
> +}
> +
> void vhost_dev_cleanup(struct vhost_dev *dev)
> {
> int i;
> @@ -626,6 +725,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> if (dev->vqs[i]->call_ctx)
> eventfd_ctx_put(dev->vqs[i]->call_ctx);
> vhost_vq_reset(dev, dev->vqs[i]);
> + vhost_clean_vmaps(dev->vqs[i]);
> }
> vhost_dev_free_iovecs(dev);
> if (dev->log_ctx)
> @@ -873,6 +973,14 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq,
>
> static inline int vhost_put_avail_event(struct vhost_virtqueue *vq)
> {
> + if (!vq->iotlb) {
> + struct vring_used *used = vq->used_ring.addr;
> +
> + *((__virtio16 *)&used->ring[vq->num]) =
> + cpu_to_vhost16(vq, vq->avail_idx);
> + return 0;
> + }
> +
> return vhost_put_user(vq, cpu_to_vhost16(vq, vq->avail_idx),
> vhost_avail_event(vq));
> }
> @@ -881,6 +989,13 @@ static inline int vhost_put_used(struct vhost_virtqueue *vq,
> struct vring_used_elem *head, int idx,
> int count)
> {
> + if (!vq->iotlb) {
> + struct vring_used *used = vq->used_ring.addr;
> +
> + memcpy(used->ring + idx, head, count * sizeof(*head));
> + return 0;
> + }
> +
> return vhost_copy_to_user(vq, vq->used->ring + idx, head,
> count * sizeof(*head));
> }
> @@ -888,6 +1003,13 @@ static inline int vhost_put_used(struct vhost_virtqueue *vq,
> static inline int vhost_put_used_flags(struct vhost_virtqueue *vq)
>
> {
> + if (!vq->iotlb) {
> + struct vring_used *used = vq->used_ring.addr;
> +
> + used->flags = cpu_to_vhost16(vq, vq->used_flags);
> + return 0;
> + }
> +
> return vhost_put_user(vq, cpu_to_vhost16(vq, vq->used_flags),
> &vq->used->flags);
> }
> @@ -895,6 +1017,13 @@ static inline int vhost_put_used_flags(struct vhost_virtqueue *vq)
> static inline int vhost_put_used_idx(struct vhost_virtqueue *vq)
>
> {
> + if (!vq->iotlb) {
> + struct vring_used *used = vq->used_ring.addr;
> +
> + used->idx = cpu_to_vhost16(vq, vq->last_used_idx);
> + return 0;
> + }
> +
> return vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx),
> &vq->used->idx);
> }
> @@ -926,12 +1055,26 @@ static inline int vhost_put_used_idx(struct vhost_virtqueue *vq)
> static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
> __virtio16 *idx)
> {
> + if (!vq->iotlb) {
> + struct vring_avail *avail = vq->avail_ring.addr;
> +
> + *idx = avail->idx;
> + return 0;
> + }
> +
> return vhost_get_avail(vq, *idx, &vq->avail->idx);
> }
>
> static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> __virtio16 *head, int idx)
> {
> + if (!vq->iotlb) {
> + struct vring_avail *avail = vq->avail_ring.addr;
> +
> + *head = avail->ring[idx & (vq->num - 1)];
> + return 0;
> + }
> +
> return vhost_get_avail(vq, *head,
> &vq->avail->ring[idx & (vq->num - 1)]);
> }
> @@ -939,24 +1082,52 @@ static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> static inline int vhost_get_avail_flags(struct vhost_virtqueue *vq,
> __virtio16 *flags)
> {
> + if (!vq->iotlb) {
> + struct vring_avail *avail = vq->avail_ring.addr;
> +
> + *flags = avail->flags;
> + return 0;
> + }
> +
> return vhost_get_avail(vq, *flags, &vq->avail->flags);
> }
>
> static inline int vhost_get_used_event(struct vhost_virtqueue *vq,
> __virtio16 *event)
> {
> + if (!vq->iotlb) {
> + struct vring_avail *avail = vq->avail_ring.addr;
> +
> + *event = (__virtio16)avail->ring[vq->num];
> + return 0;
> + }
> +
> return vhost_get_avail(vq, *event, vhost_used_event(vq));
> }
>
> static inline int vhost_get_used_idx(struct vhost_virtqueue *vq,
> __virtio16 *idx)
> {
> + if (!vq->iotlb) {
> + struct vring_used *used = vq->used_ring.addr;
> +
> + *idx = used->idx;
> + return 0;
> + }
> +
> return vhost_get_used(vq, *idx, &vq->used->idx);
> }
>
> static inline int vhost_get_desc(struct vhost_virtqueue *vq,
> struct vring_desc *desc, int idx)
> {
> + if (!vq->iotlb) {
> + struct vring_desc *d = vq->desc_ring.addr;
> +
> + *desc = *(d + idx);
> + return 0;
> + }
> +
> return vhost_copy_from_user(vq, desc, vq->desc + idx, sizeof(*desc));
> }
>
> @@ -1551,6 +1722,13 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> }
> }
>
> + if (!vq->iotlb && vhost_setup_vmaps(vq, a.avail_user_addr,
> + a.desc_user_addr,
> + a.used_user_addr)) {
> + r = -EINVAL;
> + break;
> + }
> +
> vq->log_used = !!(a.flags & (0x1 << VHOST_VRING_F_LOG));
> vq->desc = (void __user *)(unsigned long)a.desc_user_addr;
> vq->avail = (void __user *)(unsigned long)a.avail_user_addr;
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 466ef7542291..89dc0ad3d055 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -80,6 +80,12 @@ enum vhost_uaddr_type {
> VHOST_NUM_ADDRS = 3,
> };
>
> +struct vhost_vmap {
> + struct page **pages;
> + void *addr;
> + int npages;
> +};
> +
> /* The virtqueue structure describes a queue attached to a device. */
> struct vhost_virtqueue {
> struct vhost_dev *dev;
> @@ -90,6 +96,11 @@ struct vhost_virtqueue {
> struct vring_desc __user *desc;
> struct vring_avail __user *avail;
> struct vring_used __user *used;
> +
> + struct vhost_vmap avail_ring;
> + struct vhost_vmap desc_ring;
> + struct vhost_vmap used_ring;
> +
> const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
> struct file *kick;
> struct eventfd_ctx *call_ctx;
> --
> 2.17.1

Next message: Lance Richardson: "Re: Can we drop upstream Linux x32 support?"
Previous message: Jerome Brunet: "[PATCH] ASoC: meson: fix do_div warning in spdifin"
In reply to: Jason Wang: "[PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address"
Next in thread: Konrad Rzeszutek Wilk: "Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address"
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