virtio: harsher barriers for rpmsg.

We were cheating with our barriers; using the smp ones rather than the
real device ones.  That was fine, until rpmsg came along, which is
used to talk to a real device (a non-SMP CPU).

Unfortunately, just putting back the real barriers (reverting
d57ed95d) causes a performance regression on virtio-pci.  In
particular, Amos reports netbench's TCP_RR over virtio_net CPU
utilization increased up to 35% while throughput went down by up to
14%.

By comparison, this branch is in the noise.

Reference: https://lkml.org/lkml/2011/12/11/22

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index 595d7319701680d9360e9f252d9f0273e7de8726..6a1d6447b864d527ed0db5d5aca6ce67703ac227 100644 (file)
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -292,10 +292,12 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 
        /*
         * OK, tell virtio_ring.c to set up a virtqueue now we know its size
-        * and we've got a pointer to its pages.
+        * and we've got a pointer to its pages.  Note that we set weak_barriers
+        * to 'true': the host just a(nother) SMP CPU, so we only need inter-cpu
+        * barriers.
         */
-       vq = vring_new_virtqueue(lvq->config.num, LGUEST_VRING_ALIGN,
-                                vdev, lvq->pages, lg_notify, callback, name);
+       vq = vring_new_virtqueue(lvq->config.num, LGUEST_VRING_ALIGN, vdev,
+                                true, lvq->pages, lg_notify, callback, name);
        if (!vq) {
                err = -ENOMEM;
                goto unmap;

diff --git a/drivers/s390/kvm/kvm_virtio.c b/drivers/s390/kvm/kvm_virtio.c
index 8af868bab20b52f9e2c11d4266a977c79ff99c01..7bc1955337ead5eecac17adc14021f9f5f843cd6 100644 (file)
--- a/drivers/s390/kvm/kvm_virtio.c
+++ b/drivers/s390/kvm/kvm_virtio.c
@@ -198,7 +198,7 @@ static struct virtqueue *kvm_find_vq(struct virtio_device *vdev,
                goto out;
 
        vq = vring_new_virtqueue(config->num, KVM_S390_VIRTIO_RING_ALIGN,
-                                vdev, (void *) config->address,
+                                vdev, true, (void *) config->address,
                                 kvm_notify, callback, name);
        if (!vq) {
                err = -ENOMEM;

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index 0269717436af03f5d502be98629927e7591e5cc4..01d6dc250d5c50a8c01ea22d34818721d8c845d5 100644 (file)
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -310,8 +310,8 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned index,
                        vm_dev->base + VIRTIO_MMIO_QUEUE_PFN);
 
        /* Create the vring */
-       vq = vring_new_virtqueue(info->num, VIRTIO_MMIO_VRING_ALIGN,
-                                vdev, info->queue, vm_notify, callback, name);
+       vq = vring_new_virtqueue(info->num, VIRTIO_MMIO_VRING_ALIGN, vdev,
+                                true, info->queue, vm_notify, callback, name);
        if (!vq) {
                err = -ENOMEM;
                goto error_new_virtqueue;

diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index baabb7937ec2c338fc813fb91cfaac21b0c94484..688b42d28dad9124a9d5883f9e7183b7e40d1b24 100644 (file)
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -414,8 +414,8 @@ static struct virtqueue *setup_vq(struct virtio_device *vdev, unsigned index,
                  vp_dev->ioaddr + VIRTIO_PCI_QUEUE_PFN);
 
        /* create the vring */
-       vq = vring_new_virtqueue(info->num, VIRTIO_PCI_VRING_ALIGN,
-                                vdev, info->queue, vp_notify, callback, name);
+       vq = vring_new_virtqueue(info->num, VIRTIO_PCI_VRING_ALIGN, vdev,
+                                true, info->queue, vp_notify, callback, name);
        if (!vq) {
                err = -ENOMEM;
                goto out_activate_queue;

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c7a2c208f6eaded137c1f0d9ea73ea0a14100e65..50da92046092f20504b8e668ae7acf2e0759373a 100644 (file)
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -28,17 +28,20 @@
 #ifdef CONFIG_SMP
 /* Where possible, use SMP barriers which are more lightweight than mandatory
  * barriers, because mandatory barriers control MMIO effects on accesses
- * through relaxed memory I/O windows (which virtio does not use). */
-#define virtio_mb() smp_mb()
-#define virtio_rmb() smp_rmb()
-#define virtio_wmb() smp_wmb()
+ * through relaxed memory I/O windows (which virtio-pci does not use). */
+#define virtio_mb(vq) \
+       do { if ((vq)->weak_barriers) smp_mb(); else mb(); } while(0)
+#define virtio_rmb(vq) \
+       do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
+#define virtio_wmb(vq) \
+       do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
 #else
 /* We must force memory ordering even if guest is UP since host could be
  * running on another CPU, but SMP barriers are defined to barrier() in that
  * configuration. So fall back to mandatory barriers instead. */
-#define virtio_mb() mb()
-#define virtio_rmb() rmb()
-#define virtio_wmb() wmb()
+#define virtio_mb(vq) mb()
+#define virtio_rmb(vq) rmb()
+#define virtio_wmb(vq) wmb()
 #endif
 
 #ifdef DEBUG
@@ -77,6 +80,9 @@ struct vring_virtqueue
        /* Actual memory layout for this queue */
        struct vring vring;
 
+       /* Can we use weak barriers? */
+       bool weak_barriers;
+
        /* Other side has made a mess, don't try any more. */
        bool broken;
 
@@ -245,14 +251,14 @@ void virtqueue_kick(struct virtqueue *_vq)
        START_USE(vq);
        /* Descriptors and available array need to be set before we expose the
         * new available array entries. */
-       virtio_wmb();
+       virtio_wmb(vq);
 
        old = vq->vring.avail->idx;
        new = vq->vring.avail->idx = old + vq->num_added;
        vq->num_added = 0;
 
        /* Need to update avail index before checking if we should notify */
-       virtio_mb();
+       virtio_mb(vq);
 
        if (vq->event ?
            vring_need_event(vring_avail_event(&vq->vring), new, old) :
@@ -314,7 +320,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
        }
 
        /* Only get used array entries after they have been exposed by host. */
-       virtio_rmb();
+       virtio_rmb(vq);
 
        i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
        *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
@@ -337,7 +343,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
         * the read in the next get_buf call. */
        if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
                vring_used_event(&vq->vring) = vq->last_used_idx;
-               virtio_mb();
+               virtio_mb(vq);
        }
 
        END_USE(vq);
@@ -366,7 +372,7 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
         * entry. Always do both to keep code simple. */
        vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
        vring_used_event(&vq->vring) = vq->last_used_idx;
-       virtio_mb();
+       virtio_mb(vq);
        if (unlikely(more_used(vq))) {
                END_USE(vq);
                return false;
@@ -393,7 +399,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
        /* TODO: tune this threshold */
        bufs = (u16)(vq->vring.avail->idx - vq->last_used_idx) * 3 / 4;
        vring_used_event(&vq->vring) = vq->last_used_idx + bufs;
-       virtio_mb();
+       virtio_mb(vq);
        if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) {
                END_USE(vq);
                return false;
@@ -453,6 +459,7 @@ EXPORT_SYMBOL_GPL(vring_interrupt);
 struct virtqueue *vring_new_virtqueue(unsigned int num,
                                      unsigned int vring_align,
                                      struct virtio_device *vdev,
+                                     bool weak_barriers,
                                      void *pages,
                                      void (*notify)(struct virtqueue *),
                                      void (*callback)(struct virtqueue *),
@@ -476,6 +483,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
        vq->vq.vdev = vdev;
        vq->vq.name = name;
        vq->notify = notify;
+       vq->weak_barriers = weak_barriers;
        vq->broken = false;
        vq->last_used_idx = 0;
        vq->num_added = 0;

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 36be0f6e18a9cf9608c4288cb3343f618e329c87..e338730c2660f9e6efb593858bc672c486c8f996 100644 (file)
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -168,6 +168,7 @@ struct virtqueue;
 struct virtqueue *vring_new_virtqueue(unsigned int num,
                                      unsigned int vring_align,
                                      struct virtio_device *vdev,
+                                     bool weak_barriers,
                                      void *pages,
                                      void (*notify)(struct virtqueue *vq),
                                      void (*callback)(struct virtqueue *vq),

diff --git a/tools/virtio/linux/virtio.h b/tools/virtio/linux/virtio.h
index 669bcdd45805a7039473d1acd425effd6d77797c..953db2abf6b9607694e0fa71987575cc4fee07e1 100644 (file)
--- a/tools/virtio/linux/virtio.h
+++ b/tools/virtio/linux/virtio.h
@@ -214,6 +214,7 @@ void *virtqueue_detach_unused_buf(struct virtqueue *vq);
 struct virtqueue *vring_new_virtqueue(unsigned int num,
                                      unsigned int vring_align,
                                      struct virtio_device *vdev,
+                                     bool weak_barriers,
                                      void *pages,
                                      void (*notify)(struct virtqueue *vq),
                                      void (*callback)(struct virtqueue *vq),

diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
index 74d3331bdaf992b10bdf38d630d0dca00cc08360..0740284396c146b230d3ca353b20ea6f2ed6664e 100644 (file)
--- a/tools/virtio/virtio_test.c
+++ b/tools/virtio/virtio_test.c
@@ -92,7 +92,8 @@ static void vq_info_add(struct vdev_info *dev, int num)
        assert(r >= 0);
        memset(info->ring, 0, vring_size(num, 4096));
        vring_init(&info->vring, num, info->ring, 4096);
-       info->vq = vring_new_virtqueue(info->vring.num, 4096, &dev->vdev, info->ring,
+       info->vq = vring_new_virtqueue(info->vring.num, 4096, &dev->vdev,
+                                      true, info->ring,
                                       vq_notify, vq_callback, "test");
        assert(info->vq);
        info->vq->priv = info;