vhost: access vq metadata through kernel virtual address

It was noticed that the copy_to/from_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 checks,
speculation barriers, hardware feature toggling (e.g SMAP). The
extra cost will be more obvious when transferring small packets since
the time spent on metadata accessing become more significant.

This patch tries to eliminate those overheads by accessing them
through direct mapping of those pages. Invalidation callbacks is
implemented for co-operation with general VM management (swap, KSM,
THP or NUMA balancing). We will try to get the direct mapping of vq
metadata before each round of packet processing if it doesn't
exist. If we fail, we will simplely fallback to copy_to/from_user()
friends.

This invalidation and direct mapping access are synchronized through
spinlock and RCU. All matedata accessing through direct map is
protected by RCU, and the setup or invalidation are done under
spinlock.

This method might does not work for high mem page which requires
temporary mapping so we just fallback to normal
copy_to/from_user() and may not for arch that has virtual tagged cache
since extra cache flushing is needed to eliminate the alias. This will
result complex logic and bad performance. For those archs, this patch
simply go for copy_to/from_user() friends. This is done by ruling out
kernel mapping codes through ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE.

Note that this is only done when device IOTLB is not enabled. We
could use similar method to optimize IOTLB in the future.

Tests shows at most about 23% improvement on TX PPS when using
virtio-user + vhost_net + xdp1 + TAP on 2.6GHz Broadwell:

        SMAP on | SMAP off
Before: 5.2Mpps | 7.1Mpps
After:  6.4Mpps | 8.2Mpps

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Miller <davem@davemloft.net>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: linux-mm@kvack.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 512 insertions, 3 deletions

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index e27d1da5f979..dc9301d31f12 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -299,6 +299,160 @@ static void vhost_vq_meta_reset(struct vhost_dev *d)
                 __vhost_vq_meta_reset(d->vqs[i]);
 }
 
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+static void vhost_map_unprefetch(struct vhost_map *map)
+{
+        kfree(map->pages);
+        map->pages = NULL;
+        map->npages = 0;
+        map->addr = NULL;
+}
+
+static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
+{
+        struct vhost_map *map[VHOST_NUM_ADDRS];
+        int i;
+
+        spin_lock(&vq->mmu_lock);
+        for (i = 0; i < VHOST_NUM_ADDRS; i++) {
+                map[i] = rcu_dereference_protected(vq->maps[i],
+                                  lockdep_is_held(&vq->mmu_lock));
+                if (map[i])
+                        rcu_assign_pointer(vq->maps[i], NULL);
+        }
+        spin_unlock(&vq->mmu_lock);
+
+        synchronize_rcu();
+
+        for (i = 0; i < VHOST_NUM_ADDRS; i++)
+                if (map[i])
+                        vhost_map_unprefetch(map[i]);
+
+}
+
+static void vhost_reset_vq_maps(struct vhost_virtqueue *vq)
+{
+        int i;
+
+        vhost_uninit_vq_maps(vq);
+        for (i = 0; i < VHOST_NUM_ADDRS; i++)
+                vq->uaddrs[i].size = 0;
+}
+
+static bool vhost_map_range_overlap(struct vhost_uaddr *uaddr,
+                                     unsigned long start,
+                                     unsigned long end)
+{
+        if (unlikely(!uaddr->size))
+                return false;
+
+        return !(end < uaddr->uaddr || start > uaddr->uaddr - 1 + uaddr->size);
+}
+
+static void vhost_invalidate_vq_start(struct vhost_virtqueue *vq,
+                                      int index,
+                                      unsigned long start,
+                                      unsigned long end)
+{
+        struct vhost_uaddr *uaddr = &vq->uaddrs[index];
+        struct vhost_map *map;
+        int i;
+
+        if (!vhost_map_range_overlap(uaddr, start, end))
+                return;
+
+        spin_lock(&vq->mmu_lock);
+        ++vq->invalidate_count;
+
+        map = rcu_dereference_protected(vq->maps[index],
+                                        lockdep_is_held(&vq->mmu_lock));
+        if (map) {
+                if (uaddr->write) {
+                        for (i = 0; i < map->npages; i++)
+                                set_page_dirty(map->pages[i]);
+                }
+                rcu_assign_pointer(vq->maps[index], NULL);
+        }
+        spin_unlock(&vq->mmu_lock);
+
+        if (map) {
+                synchronize_rcu();
+                vhost_map_unprefetch(map);
+        }
+}
+
+static void vhost_invalidate_vq_end(struct vhost_virtqueue *vq,
+                                    int index,
+                                    unsigned long start,
+                                    unsigned long end)
+{
+        if (!vhost_map_range_overlap(&vq->uaddrs[index], start, end))
+                return;
+
+        spin_lock(&vq->mmu_lock);
+        --vq->invalidate_count;
+        spin_unlock(&vq->mmu_lock);
+}
+
+static int vhost_invalidate_range_start(struct mmu_notifier *mn,
+                                        const struct mmu_notifier_range *range)
+{
+        struct vhost_dev *dev = container_of(mn, struct vhost_dev,
+                                             mmu_notifier);
+        int i, j;
+
+        if (!mmu_notifier_range_blockable(range))
+                return -EAGAIN;
+
+        for (i = 0; i < dev->nvqs; i++) {
+                struct vhost_virtqueue *vq = dev->vqs[i];
+
+                for (j = 0; j < VHOST_NUM_ADDRS; j++)
+                        vhost_invalidate_vq_start(vq, j,
+                                                  range->start,
+                                                  range->end);
+        }
+
+        return 0;
+}
+
+static void vhost_invalidate_range_end(struct mmu_notifier *mn,
+                                       const struct mmu_notifier_range *range)
+{
+        struct vhost_dev *dev = container_of(mn, struct vhost_dev,
+                                             mmu_notifier);
+        int i, j;
+
+        for (i = 0; i < dev->nvqs; i++) {
+                struct vhost_virtqueue *vq = dev->vqs[i];
+
+                for (j = 0; j < VHOST_NUM_ADDRS; j++)
+                        vhost_invalidate_vq_end(vq, j,
+                                                range->start,
+                                                range->end);
+        }
+}
+
+static const struct mmu_notifier_ops vhost_mmu_notifier_ops = {
+        .invalidate_range_start = vhost_invalidate_range_start,
+        .invalidate_range_end = vhost_invalidate_range_end,
+};
+
+static void vhost_init_maps(struct vhost_dev *dev)
+{
+        struct vhost_virtqueue *vq;
+        int i, j;
+
+        dev->mmu_notifier.ops = &vhost_mmu_notifier_ops;
+
+        for (i = 0; i < dev->nvqs; ++i) {
+                vq = dev->vqs[i];
+                for (j = 0; j < VHOST_NUM_ADDRS; j++)
+                        RCU_INIT_POINTER(vq->maps[j], NULL);
+        }
+}
+#endif
+
 static void vhost_vq_reset(struct vhost_dev *dev,
                            struct vhost_virtqueue *vq)
 {
@@ -327,7 +481,11 @@ static void vhost_vq_reset(struct vhost_dev *dev,
         vq->busyloop_timeout = 0;
         vq->umem = NULL;
         vq->iotlb = NULL;
+        vq->invalidate_count = 0;
         __vhost_vq_meta_reset(vq);
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        vhost_reset_vq_maps(vq);
+#endif
 }
 
 static int vhost_worker(void *data)
@@ -477,7 +635,9 @@ void vhost_dev_init(struct vhost_dev *dev,
         INIT_LIST_HEAD(&dev->read_list);
         INIT_LIST_HEAD(&dev->pending_list);
         spin_lock_init(&dev->iotlb_lock);
-
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        vhost_init_maps(dev);
+#endif
 
         for (i = 0; i < dev->nvqs; ++i) {
                 vq = dev->vqs[i];
@@ -486,6 +646,7 @@ void vhost_dev_init(struct vhost_dev *dev,
                 vq->heads = NULL;
                 vq->dev = dev;
                 mutex_init(&vq->mutex);
+                spin_lock_init(&vq->mmu_lock);
                 vhost_vq_reset(dev, vq);
                 if (vq->handle_kick)
                         vhost_poll_init(&vq->poll, vq->handle_kick,
@@ -565,7 +726,18 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
         if (err)
                 goto err_cgroup;
 
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        err = mmu_notifier_register(&dev->mmu_notifier, dev->mm);
+        if (err)
+                goto err_mmu_notifier;
+#endif
+
         return 0;
+
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+err_mmu_notifier:
+        vhost_dev_free_iovecs(dev);
+#endif
 err_cgroup:
         kthread_stop(worker);
         dev->worker = NULL;
@@ -656,6 +828,107 @@ static void vhost_clear_msg(struct vhost_dev *dev)
         spin_unlock(&dev->iotlb_lock);
 }
 
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+static void vhost_setup_uaddr(struct vhost_virtqueue *vq,
+                              int index, unsigned long uaddr,
+                              size_t size, bool write)
+{
+        struct vhost_uaddr *addr = &vq->uaddrs[index];
+
+        addr->uaddr = uaddr;
+        addr->size = size;
+        addr->write = write;
+}
+
+static void vhost_setup_vq_uaddr(struct vhost_virtqueue *vq)
+{
+        vhost_setup_uaddr(vq, VHOST_ADDR_DESC,
+                          (unsigned long)vq->desc,
+                          vhost_get_desc_size(vq, vq->num),
+                          false);
+        vhost_setup_uaddr(vq, VHOST_ADDR_AVAIL,
+                          (unsigned long)vq->avail,
+                          vhost_get_avail_size(vq, vq->num),
+                          false);
+        vhost_setup_uaddr(vq, VHOST_ADDR_USED,
+                          (unsigned long)vq->used,
+                          vhost_get_used_size(vq, vq->num),
+                          true);
+}
+
+static int vhost_map_prefetch(struct vhost_virtqueue *vq,
+                               int index)
+{
+        struct vhost_map *map;
+        struct vhost_uaddr *uaddr = &vq->uaddrs[index];
+        struct page **pages;
+        int npages = DIV_ROUND_UP(uaddr->size, PAGE_SIZE);
+        int npinned;
+        void *vaddr, *v;
+        int err;
+        int i;
+
+        spin_lock(&vq->mmu_lock);
+
+        err = -EFAULT;
+        if (vq->invalidate_count)
+                goto err;
+
+        err = -ENOMEM;
+        map = kmalloc(sizeof(*map), GFP_ATOMIC);
+        if (!map)
+                goto err;
+
+        pages = kmalloc_array(npages, sizeof(struct page *), GFP_ATOMIC);
+        if (!pages)
+                goto err_pages;
+
+        err = EFAULT;
+        npinned = __get_user_pages_fast(uaddr->uaddr, npages,
+                                        uaddr->write, pages);
+        if (npinned > 0)
+                release_pages(pages, npinned);
+        if (npinned != npages)
+                goto err_gup;
+
+        for (i = 0; i < npinned; i++)
+                if (PageHighMem(pages[i]))
+                        goto err_gup;
+
+        vaddr = v = page_address(pages[0]);
+
+        /* For simplicity, fallback to userspace address if VA is not
+         * contigious.
+         */
+        for (i = 1; i < npinned; i++) {
+                v += PAGE_SIZE;
+                if (v != page_address(pages[i]))
+                        goto err_gup;
+        }
+
+        map->addr = vaddr + (uaddr->uaddr & (PAGE_SIZE - 1));
+        map->npages = npages;
+        map->pages = pages;
+
+        rcu_assign_pointer(vq->maps[index], map);
+        /* No need for a synchronize_rcu(). This function should be
+         * called by dev->worker so we are serialized with all
+         * readers.
+         */
+        spin_unlock(&vq->mmu_lock);
+
+        return 0;
+
+err_gup:
+        kfree(pages);
+err_pages:
+        kfree(map);
+err:
+        spin_unlock(&vq->mmu_lock);
+        return err;
+}
+#endif
+
 void vhost_dev_cleanup(struct vhost_dev *dev)
 {
         int i;
@@ -685,8 +958,16 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
                 kthread_stop(dev->worker);
                 dev->worker = NULL;
         }
-        if (dev->mm)
+        if (dev->mm) {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+                mmu_notifier_unregister(&dev->mmu_notifier, dev->mm);
+#endif
                 mmput(dev->mm);
+        }
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        for (i = 0; i < dev->nvqs; i++)
+                vhost_uninit_vq_maps(dev->vqs[i]);
+#endif
         dev->mm = NULL;
 }
 EXPORT_SYMBOL_GPL(vhost_dev_cleanup);
@@ -915,6 +1196,26 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq,
 
 static inline int vhost_put_avail_event(struct vhost_virtqueue *vq)
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_used *used;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_USED]);
+                if (likely(map)) {
+                        used = map->addr;
+                        *((__virtio16 *)&used->ring[vq->num]) =
+                                cpu_to_vhost16(vq, vq->avail_idx);
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_put_user(vq, cpu_to_vhost16(vq, vq->avail_idx),
                               vhost_avail_event(vq));
 }
@@ -923,6 +1224,27 @@ static inline int vhost_put_used(struct vhost_virtqueue *vq,
                                  struct vring_used_elem *head, int idx,
                                  int count)
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_used *used;
+        size_t size;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_USED]);
+                if (likely(map)) {
+                        used = map->addr;
+                        size = count * sizeof(*head);
+                        memcpy(used->ring + idx, head, size);
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_copy_to_user(vq, vq->used->ring + idx, head,
                                   count * sizeof(*head));
 }
@@ -930,6 +1252,25 @@ static inline int vhost_put_used(struct vhost_virtqueue *vq,
 static inline int vhost_put_used_flags(struct vhost_virtqueue *vq)
 
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_used *used;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_USED]);
+                if (likely(map)) {
+                        used = map->addr;
+                        used->flags = cpu_to_vhost16(vq, vq->used_flags);
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_put_user(vq, cpu_to_vhost16(vq, vq->used_flags),
                               &vq->used->flags);
 }
@@ -937,6 +1278,25 @@ static inline int vhost_put_used_flags(struct vhost_virtqueue *vq)
 static inline int vhost_put_used_idx(struct vhost_virtqueue *vq)
 
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_used *used;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_USED]);
+                if (likely(map)) {
+                        used = map->addr;
+                        used->idx = cpu_to_vhost16(vq, vq->last_used_idx);
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx),
                               &vq->used->idx);
 }
@@ -982,12 +1342,50 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
 static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
                                       __virtio16 *idx)
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_avail *avail;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_AVAIL]);
+                if (likely(map)) {
+                        avail = map->addr;
+                        *idx = avail->idx;
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         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 VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_avail *avail;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_AVAIL]);
+                if (likely(map)) {
+                        avail = map->addr;
+                        *head = avail->ring[idx & (vq->num - 1)];
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_get_avail(vq, *head,
                                &vq->avail->ring[idx & (vq->num - 1)]);
 }
@@ -995,24 +1393,98 @@ 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 VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_avail *avail;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_AVAIL]);
+                if (likely(map)) {
+                        avail = map->addr;
+                        *flags = avail->flags;
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_get_avail(vq, *flags, &vq->avail->flags);
 }
 
 static inline int vhost_get_used_event(struct vhost_virtqueue *vq,
                                        __virtio16 *event)
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_avail *avail;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+                map = rcu_dereference(vq->maps[VHOST_ADDR_AVAIL]);
+                if (likely(map)) {
+                        avail = map->addr;
+                        *event = (__virtio16)avail->ring[vq->num];
+                        rcu_read_unlock();
+                        return 0;
+                }
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_get_avail(vq, *event, vhost_used_event(vq));
 }
 
 static inline int vhost_get_used_idx(struct vhost_virtqueue *vq,
                                      __virtio16 *idx)
 {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_used *used;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_USED]);
+                if (likely(map)) {
+                        used = map->addr;
+                        *idx = used->idx;
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         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 VHOST_ARCH_CAN_ACCEL_UACCESS
+        struct vhost_map *map;
+        struct vring_desc *d;
+
+        if (!vq->iotlb) {
+                rcu_read_lock();
+
+                map = rcu_dereference(vq->maps[VHOST_ADDR_DESC]);
+                if (likely(map)) {
+                        d = map->addr;
+                        *desc = *(d + idx);
+                        rcu_read_unlock();
+                        return 0;
+                }
+
+                rcu_read_unlock();
+        }
+#endif
+
         return vhost_copy_from_user(vq, desc, vq->desc + idx, sizeof(*desc));
 }
 
@@ -1353,12 +1825,32 @@ static bool iotlb_access_ok(struct vhost_virtqueue *vq,
         return true;
 }
 
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+static void vhost_vq_map_prefetch(struct vhost_virtqueue *vq)
+{
+        struct vhost_map __rcu *map;
+        int i;
+
+        for (i = 0; i < VHOST_NUM_ADDRS; i++) {
+                rcu_read_lock();
+                map = rcu_dereference(vq->maps[i]);
+                rcu_read_unlock();
+                if (unlikely(!map))
+                        vhost_map_prefetch(vq, i);
+        }
+}
+#endif
+
 int vq_meta_prefetch(struct vhost_virtqueue *vq)
 {
         unsigned int num = vq->num;
 
-        if (!vq->iotlb)
+        if (!vq->iotlb) {
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+                vhost_vq_map_prefetch(vq);
+#endif
                 return 1;
+        }
 
         return iotlb_access_ok(vq, VHOST_ACCESS_RO, (u64)(uintptr_t)vq->desc,
                                vhost_get_desc_size(vq, num), VHOST_ADDR_DESC) &&
@@ -1569,6 +2061,16 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
 
         mutex_lock(&vq->mutex);
 
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        /* Unregister MMU notifer to allow invalidation callback
+         * can access vq->uaddrs[] without holding a lock.
+         */
+        if (d->mm)
+                mmu_notifier_unregister(&d->mmu_notifier, d->mm);
+
+        vhost_uninit_vq_maps(vq);
+#endif
+
         switch (ioctl) {
         case VHOST_SET_VRING_NUM:
                 r = vhost_vring_set_num(d, vq, argp);
@@ -1580,6 +2082,13 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
                 BUG();
         }
 
+#if VHOST_ARCH_CAN_ACCEL_UACCESS
+        vhost_setup_vq_uaddr(vq);
+
+        if (d->mm)
+                mmu_notifier_register(&d->mmu_notifier, d->mm);
+#endif
+
         mutex_unlock(&vq->mutex);
 
         return r;