On 10/15/2014 07:38 PM, Michael S. Tsirkin wrote: > On Wed, Oct 15, 2014 at 06:44:41PM +0800, Jason Wang wrote: >> On 10/15/2014 06:32 PM, Michael S. Tsirkin wrote: >>> On Wed, Oct 15, 2014 at 06:13:19PM +0800, Jason Wang wrote: >>>> On 10/15/2014 05:34 PM, Michael S. Tsirkin wrote: >>>>> On Wed, Oct 15, 2014 at 03:25:25PM +0800, Jason Wang wrote: >>>>>> This patch checks the new event idx to make sure used event idx never >>>>>> goes back. This is used to synchronize the calls between >>>>>> virtqueue_enable_cb_delayed() and virtqueue_enable_cb(). >>>>>> >>>>>> Cc: Rusty Russell <ru...@rustcorp.com.au> >>>>>> Cc: Michael S. Tsirkin <m...@redhat.com> >>>>>> Signed-off-by: Jason Wang <jasow...@redhat.com> >>>>> the implication being that moving event idx back might cause some race >>>>> condition? >>>> This will cause race condition when tx interrupt is enabled. Consider >>>> the following cases >>>> >>>> 1) tx napi was scheduled >>>> 2) start_xmit() call virtqueue_enable_cb_delayed() and disable cb, [used >>>> event is vq->last_used_idx + 3/4 pendg bufs] >>>> 3) tx napi enable the callback by virtqueue_enable_cb() [ used event is >>>> vq->last_used_idx ] >>>> >>>> After step 3, used event was moved back, unnecessary tx interrupt was >>>> triggered. >>> Well unnecessary interrupts are safe. >> But it that is what we want to reduce. > It's all about correctness. I don't think mixing enable_cb > and enable_cb_delayed makes sense, let's just make > virtio behave correctly if that happens, no need to > optimize for that.
Then as you said, need document or add WARN_ON() or BUG() in case both of the two are used. > > >>> With your patch caller of virtqueue_enable_cb will not get an >>> interrupt on the next buffer which is not safe. >>> >>> If you don't want an interrupt on the next buffer, don't >>> call virtqueue_enable_cb. >> So something like this patch should be done in virtio core somewhere >> else. Virtio-net can not do this since it does not have the knowledge of >> event index. > Take a look at my patch - no calls to enable_cb, only > enable_cb_delayed, so we should be fine. > >>>>> If yes but please describe the race explicitly. >>>>> Is there a bug we need to fix on stable? >>>> Looks not, current code does not have such race condition. >>>>> Please also explicitly describe a configuration that causes event idx >>>>> to go back. >>>>> >>>>> All this info should go in the commit log. >>>> Will do this. >>>>>> --- >>>>>> drivers/virtio/virtio_ring.c | 7 +++++-- >>>>>> 1 files changed, 5 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c >>>>>> index 3b1f89b..1b3929f 100644 >>>>>> --- a/drivers/virtio/virtio_ring.c >>>>>> +++ b/drivers/virtio/virtio_ring.c >>>>>> @@ -559,14 +559,17 @@ unsigned virtqueue_enable_cb_prepare(struct >>>>>> virtqueue *_vq) >>>>>> u16 last_used_idx; >>>>>> >>>>>> START_USE(vq); >>>>>> - >>>>>> + last_used_idx = vq->last_used_idx; >>>>>> /* We optimistically turn back on interrupts, then check if >>>>>> there was >>>>>> * more to do. */ >>>>>> /* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to >>>>>> * either clear the flags bit or point the event index at the >>>>>> next >>>>>> * entry. Always do both to keep code simple. */ >>>>>> vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT; >>>>>> - vring_used_event(&vq->vring) = last_used_idx = >>>>>> vq->last_used_idx; >>>>>> + /* Make sure used event never go backwards */ >>>>> s/go/goes/ >>>>> >>>>>> + if (!vring_need_event(vring_used_event(&vq->vring), >>>>>> + vq->vring.avail->idx, last_used_idx)) >>>>>> + vring_used_event(&vq->vring) = last_used_idx; >>>>> The result will be that driver will *not* get an interrupt >>>>> on the next consumed buffer, which is likely not what driver >>>>> intended when it called virtqueue_enable_cb. >>>> This will only happen when we want to delay the interrupt for next few >>>> consumed buffers (virtqueue_enable_cb_delayed() was called). For the >>>> other case, vq->last_used_idx should be ahead of previous used event. Do >>>> you see any other case? >>> Call virtqueue_enable_cb_delayed, later call virtqueue_enable_cb. If >>> event index is not updated in virtqueue_enable_cb, driver will not get >>> an interrupt on the next buffer. >> This is just what we want I think. The interrupt was not lost but fired >> after 3/4 pending buffers were consumed. Do you see any real issue on this? > Yes, this violates the API. For example device might never > consume the rest of buffers. Then it should be a bug of device which is out of the control of guest. If not, device might never also consume 3/4 rest of buffers. > >>>>> Instead, how about we simply document the requirement that drivers either >>>>> always call virtqueue_enable_cb_delayed or virtqueue_enable_cb >>>>> but not both? >>>> We need call them both when tx interrupt is enabled I believe. >>> Can you pls reply to my patch and document issues you see? >>> >> In the previous reply you said you're using >> virtuqueue_enable_cb_delayed(), so no race in your patch. > OK so you think my patch is also correct, but that yours gives better > efficiency? > Need some benchmark to see the difference I think. _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
