Re: [Qemu-devel] [PATCH v4 08/11] virtio: event suppression support for packed ring

[Wei Xu <wexu@redhat.com>]

On Tue, Feb 19, 2019 at 09:06:42PM +0800, Jason Wang wrote:
> 
> On 2019/2/19 下午6:40, Wei Xu wrote:
> >On Tue, Feb 19, 2019 at 03:19:58PM +0800, Jason Wang wrote:
> >>On 2019/2/14 下午12:26, wexu@redhat.com wrote:
> >>>From: Wei Xu <wexu@redhat.com>
> >>>
> >>>Difference between 'avail_wrap_counter' and 'last_avail_wrap_counter':
> >>>For Tx(guest transmitting), they are the same after each pop of a desc.
> >>>
> >>>For Rx(guest receiving), they are also the same when there are enough
> >>>descriptors to carry the payload for a packet(e.g. usually 16 descs are
> >>>needed for a 64k packet in typical iperf tcp connection with tso enabled),
> >>>however, when the ring is running out of descriptors while there are
> >>>still a few free ones, e.g. 6 descriptors are available which is not
> >>>enough to carry an entire packet which needs 16 descriptors, in this
> >>>case the 'avail_wrap_counter' should be set as the first one pending
> >>>being handled by guest driver in order to get a notification, and the
> >>>'last_avail_wrap_counter' should stay unchanged to the head of available
> >>>descriptors, like below:
> >>>
> >>>Mark meaning:
> >>>     | | -- available
> >>>     |*| -- used
> >>>
> >>>A Snapshot of the queue:
> >>>                                       last_avail_idx = 253
> >>>                                       last_avail_wrap_counter = 1
> >>>                                              |
> >>>     +---------------------------------------------+
> >>>  0  | | | |*|*|*|*|*|*|*|*|*|*|*|*|*|*|*|*|*| | | | 255
> >>>     +---------------------------------------------+
> >>>            |
> >>>   shadow_avail_idx = 3
> >>>   avail_wrap_counter = 0
> >>
> >>Well this might not be the good place to describe the difference between
> >>shadow_avail_idx and last_avail_idx. And the comments above their definition
> >>looks good enough?
> >Sorry, I do not get it, can you elaborate more?
> 
> 
> I meant the comment is good enough to explain what it did. For packed ring,
> the only difference is the wrap counter. You can add e.g "The wrap counter
> of next head to pop" to above last_avail_wrap_counter. And so does
> shadow_avail_wrap_counter.

OK, I will remove the example part.

> 
> 
> >
> >This is one of the buggy parts of packed ring, it is good to make it clear here.
> >
> >>     /* Next head to pop */
> >>     uint16_t last_avail_idx;
> >>
> >>     /* Last avail_idx read from VQ. */
> >>     uint16_t shadow_avail_idx;
> >>
> >What is the meaning of these comment?
> 
> 
> It's pretty easy to be understood, isn't it?

:)

> 
> 
> >  Do you mean I should replace put them
> >to the comments also? thanks.
> >
> >>Instead, how or why need event suppress is not mentioned ...
> >Yes, but presumably this knowledge has been acquired from reading through the
> >spec, so I skipped this part.
> 
> 
> You need at least add reference to the spec. Commit log is pretty important
> for starters to understand what has been done in the patch before reading
> the code. I'm pretty sure they will get confused for reading what you wrote
> here.
> 

OK.

> 
> Thanks
> 
> 
> >
> >Wei
> >
> >>
> >>
> >>>Signed-off-by: Wei Xu <wexu@redhat.com>
> >>>---
> >>>  hw/virtio/virtio.c | 137 +++++++++++++++++++++++++++++++++++++++++++++++++----
> >>>  1 file changed, 128 insertions(+), 9 deletions(-)
> >>>
> >>>diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >>>index 7e276b4..8cfc7b6 100644
> >>>--- a/hw/virtio/virtio.c
> >>>+++ b/hw/virtio/virtio.c
> >>>@@ -234,6 +234,34 @@ static void vring_desc_read(VirtIODevice *vdev, VRingDesc *desc,
> >>>      virtio_tswap16s(vdev, &desc->next);
> >>>  }
> >>>+static void vring_packed_event_read(VirtIODevice *vdev,
> >>>+                            MemoryRegionCache *cache, VRingPackedDescEvent *e)
> >>>+{
> >>>+    address_space_read_cached(cache, 0, e, sizeof(*e));
> >>>+    virtio_tswap16s(vdev, &e->off_wrap);
> >>>+    virtio_tswap16s(vdev, &e->flags);
> >>>+}
> >>>+
> >>>+static void vring_packed_off_wrap_write(VirtIODevice *vdev,
> >>>+                            MemoryRegionCache *cache, uint16_t off_wrap)
> >>>+{
> >>>+    virtio_tswap16s(vdev, &off_wrap);
> >>>+    address_space_write_cached(cache, offsetof(VRingPackedDescEvent, off_wrap),
> >>>+                            &off_wrap, sizeof(off_wrap));
> >>>+    address_space_cache_invalidate(cache,
> >>>+                offsetof(VRingPackedDescEvent, off_wrap), sizeof(off_wrap));
> >>>+}
> >>>+
> >>>+static void vring_packed_flags_write(VirtIODevice *vdev,
> >>>+                            MemoryRegionCache *cache, uint16_t flags)
> >>>+{
> >>>+    virtio_tswap16s(vdev, &flags);
> >>>+    address_space_write_cached(cache, offsetof(VRingPackedDescEvent, flags),
> >>>+                            &flags, sizeof(flags));
> >>>+    address_space_cache_invalidate(cache,
> >>>+                        offsetof(VRingPackedDescEvent, flags), sizeof(flags));
> >>>+}
> >>>+
> >>>  static VRingMemoryRegionCaches *vring_get_region_caches(struct VirtQueue *vq)
> >>>  {
> >>>      VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
> >>>@@ -340,14 +368,8 @@ static inline void vring_set_avail_event(VirtQueue *vq, uint16_t val)
> >>>      address_space_cache_invalidate(&caches->used, pa, sizeof(val));
> >>>  }
> >>>-void virtio_queue_set_notification(VirtQueue *vq, int enable)
> >>>+static void virtio_queue_set_notification_split(VirtQueue *vq, int enable)
> >>>  {
> >>>-    vq->notification = enable;
> >>>-
> >>>-    if (!vq->vring.desc) {
> >>>-        return;
> >>>-    }
> >>>-
> >>>      rcu_read_lock();
> >>>      if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >>>          vring_set_avail_event(vq, vring_avail_idx(vq));
> >>>@@ -363,6 +385,57 @@ void virtio_queue_set_notification(VirtQueue *vq, int enable)
> >>>      rcu_read_unlock();
> >>>  }
> >>>+static void virtio_queue_set_notification_packed(VirtQueue *vq, int enable)
> >>>+{
> >>>+    VRingPackedDescEvent e;
> >>>+    VRingMemoryRegionCaches *caches;
> >>>+
> >>>+    rcu_read_lock();
> >>>+    caches  = vring_get_region_caches(vq);
> >>>+    vring_packed_event_read(vq->vdev, &caches->used, &e);
> >>>+
> >>>+    if (!enable) {
> >>>+        if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >>>+            /* no need to write device area since this is outdated. */
> >>
> >>We should advertise off and wrap in this case as well, otherwise we may get
> >>notifications earlier than expected.
> >Is it necessary? Supposing offset & wrap_counter are always set before update
> >notification flags, it is reliable to skip this for disabling, isn't it?
> 
> 
> You should either:
> 
> - advertise the EVENT_FLAG_DISABLE
> 
> or
> 
> - advertise the new off wrap otherwise you may get notified early.
> 
> Both you none of above did by your code.

Right.

> 
> 
> >
> >While the logic here is unclear, I did a concise one like below
> >which doesn't use the 'vq->notification' as in your comment for v2,
> >I think this should work for packed ring as well, anything I missed?
> >
> >     if (!enable) {
> >         e.flags = VRING_PACKED_EVENT_FLAG_DISABLE;
> >     } else if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >         uint16_t off_wrap;
> >
> >         off_wrap = vq->shadow_avail_idx | vq->shadow_avail_wrap_counter << 15;
> >         vring_packed_off_wrap_write(vq->vdev, &caches->used, off_wrap);
> >
> >         /* Make sure off_wrap is wrote before flags */
> >         smp_wmb();
> >         e.flags = VRING_PACKED_EVENT_FLAG_DESC;
> >     } else {
> >         e.flags = VRING_PACKED_EVENT_FLAG_ENABLE;
> >     }
> >
> >     vring_packed_flags_write(vq->vdev, &caches->used, e.flags);
> 
> 
> Looks good to me.

Thanks.

> 
> Thanks
> 
> 
> >
> >>
> >>>+            goto out;
> >>>+        }
> >>>+
> >>>+        e.flags = VRING_PACKED_EVENT_FLAG_DISABLE;
> >>>+        goto update;
> >>>+    }
> >>>+
> >>>+    e.flags = VRING_PACKED_EVENT_FLAG_ENABLE;
> >>
> >>Here and the above goto could be eliminated by:
> >>
> >>if (even idx) {
> >>
> >>...
> >>
> >>} else if (enable) {
> >>
> >>...
> >>
> >>} else {
> >>
> >>...
> >>
> >>}
> >>
> >thanks, I have removed it in above snippet.
> >
> >Wei
> >>Thanks
> >>
> >>
> >>>+    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> >>>+        uint16_t off_wrap = vq->shadow_avail_idx | vq->avail_wrap_counter << 15;
> >>>+
> >>>+        vring_packed_off_wrap_write(vq->vdev, &caches->used, off_wrap);
> >>>+        /* Make sure off_wrap is wrote before flags */
> >>>+        smp_wmb();
> >>>+
> >>>+        e.flags = VRING_PACKED_EVENT_FLAG_DESC;
> >>>+    }
> >>>+
> >>>+update:
> >>>+    vring_packed_flags_write(vq->vdev, &caches->used, e.flags);
> >>>+out:
> >>>+    rcu_read_unlock();
> >>>+}
> >>>+
> >>>+void virtio_queue_set_notification(VirtQueue *vq, int enable)
> >>>+{
> >>>+    vq->notification = enable;
> >>>+
> >>>+    if (!vq->vring.desc) {
> >>>+        return;
> >>>+    }
> >>>+
> >>>+    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> >>>+        virtio_queue_set_notification_packed(vq, enable);
> >>>+    } else {
> >>>+        virtio_queue_set_notification_split(vq, enable);
> >>>+    }
> >>>+}
> >>>+
> >>>  int virtio_queue_ready(VirtQueue *vq)
> >>>  {
> >>>      return vq->vring.avail != 0;
> >>>@@ -2117,8 +2190,7 @@ static void virtio_set_isr(VirtIODevice *vdev, int value)
> >>>      }
> >>>  }
> >>>-/* Called within rcu_read_lock().  */
> >>>-static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> >>>+static bool virtio_split_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> >>>  {
> >>>      uint16_t old, new;
> >>>      bool v;
> >>>@@ -2141,6 +2213,53 @@ static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> >>>      return !v || vring_need_event(vring_get_used_event(vq), new, old);
> >>>  }
> >>>+static bool vring_packed_need_event(VirtQueue *vq, bool wrap,
> >>>+                            uint16_t off_wrap, uint16_t new, uint16_t old)
> >>>+{
> >>>+    int off = off_wrap & ~(1 << 15);
> >>>+
> >>>+    if (wrap != off_wrap >> 15) {
> >>>+        off -= vq->vring.num;
> >>>+    }
> >>>+
> >>>+    return vring_need_event(off, new, old);
> >>>+}
> >>>+
> >>>+static bool virtio_packed_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> >>>+{
> >>>+    VRingPackedDescEvent e;
> >>>+    uint16_t old, new;
> >>>+    bool v;
> >>>+    VRingMemoryRegionCaches *caches;
> >>>+
> >>>+    caches  = vring_get_region_caches(vq);
> >>>+    vring_packed_event_read(vdev, &caches->avail, &e);
> >>>+
> >>>+    old = vq->signalled_used;
> >>>+    new = vq->signalled_used = vq->used_idx;
> >>>+    v = vq->signalled_used_valid;
> >>>+    vq->signalled_used_valid = true;
> >>>+
> >>>+    if (e.flags == VRING_PACKED_EVENT_FLAG_DISABLE) {
> >>>+        return false;
> >>>+    } else if (e.flags == VRING_PACKED_EVENT_FLAG_ENABLE) {
> >>>+        return true;
> >>>+    }
> >>>+
> >>>+    return !v || vring_packed_need_event(vq,
> >>>+        vq->used_wrap_counter, e.off_wrap, new, old);
> >>>+}
> >>>+
> >>>+/* Called within rcu_read_lock().  */
> >>>+static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> >>>+{
> >>>+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> >>>+        return virtio_packed_should_notify(vdev, vq);
> >>>+    } else {
> >>>+        return virtio_split_should_notify(vdev, vq);
> >>>+    }
> >>>+}
> >>>+
> >>>  void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
> >>>  {
> >>>      bool should_notify;