On Tue, Feb 19, 2019 at 03:19:58PM +0800, Jason Wang wrote:
>
> On 2019/2/14 下午12:26, w...@redhat.com wrote:
> >From: Wei Xu <w...@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?
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? 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.
Wei
>
>
>
> >
> >Signed-off-by: Wei Xu <w...@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?
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);
>
>
> >+ 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;
>
