From: Tiwei Bie <tiwei.bie@intel.com> To: Jason Wang <jasowang@redhat.com> Cc: "Michael S. Tsirkin" <mst@redhat.com>, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, wexu@redhat.com, jfreimann@redhat.com Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring Date: Tue, 8 May 2018 17:16:28 +0800 [thread overview] Message-ID: <20180508091628.d7jzpopqopq4abhy@debian> (raw) In-Reply-To: <34f2c690-7cb2-f9ea-2ce9-40f4ccb594c9@redhat.com> On Tue, May 08, 2018 at 03:16:53PM +0800, Jason Wang wrote: > On 2018年05月08日 14:44, Tiwei Bie wrote: > > On Tue, May 08, 2018 at 01:40:40PM +0800, Jason Wang wrote: > > > On 2018年05月08日 11:05, Jason Wang wrote: > > > > > Because in virtqueue_enable_cb_delayed(), we may set an > > > > > event_off which is bigger than new and both of them have > > > > > wrapped. And in this case, although new is smaller than > > > > > event_off (i.e. the third param -- old), new shouldn't > > > > > add vq->num, and actually we are expecting a very big > > > > > idx diff. > > > > Yes, so to calculate distance correctly between event and new, we just > > > > need to compare the warp counter and return false if it doesn't match > > > > without the need to try to add vq.num here. > > > > > > > > Thanks > > > Sorry, looks like the following should work, we need add vq.num if > > > used_wrap_counter does not match: > > > > > > static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq, > > > __u16 off_wrap, __u16 new, > > > __u16 old) > > > { > > > bool wrap = off_wrap >> 15; > > > int off = off_wrap & ~(1 << 15); > > > __u16 d1, d2; > > > > > > if (wrap != vq->used_wrap_counter) > > > d1 = new + vq->num - off - 1; > > Just to draw your attention (maybe you have already > > noticed this). > > I miss this, thanks! > > > > > In this case (i.e. wrap != vq->used_wrap_counter), > > it's also possible that (off < new) is true. Because, > > > > when virtqueue_enable_cb_delayed_packed() is used, > > `off` is calculated in driver in a way like this: > > > > off = vq->last_used_idx + bufs; > > if (off >= vq->vring_packed.num) { > > off -= vq->vring_packed.num; > > wrap_counter ^= 1; > > } > > > > And when `new` (in vhost) is close to vq->num. The > > vq->last_used_idx + bufs (in driver) can be bigger > > than vq->vring_packed.num, and: > > > > 1. `off` will wrap; > > 2. wrap counters won't match; > > 3. off < new; > > > > And d1 (i.e. new + vq->num - off - 1) will be a value > > bigger than vq->num. I'm okay with this, although it's > > a bit weird. > > > So I'm considering something more compact by reusing vring_need_event() by > pretending a larger queue size and adding vq->num back when necessary: > > static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq, > __u16 off_wrap, __u16 new, > __u16 old) > { > bool wrap = vq->used_wrap_counter; If the wrap counter is obtained from the vq, I think `new` should also be obtained from the vq. Or the wrap counter should be carried in `new`. > int off = off_wrap & ~(1 << 15); > __u16 d1, d2; > > if (new < old) { > new += vq->num; > wrap ^= 1; > } > > if (wrap != off_wrap >> 15) > off += vq->num; When `new` and `old` wraps, and `off` doesn't wrap, wrap != (off_wrap >> 15) will be true. In this case, `off` is bigger than `new`, and what we should do is `off -= vq->num` instead of `off += vq->num`. Best regards, Tiwei Bie > > return vring_need_event(off, new, old); > } > > > > > > Best regards, > > Tiwei Bie > > > > > else > > > d1 = new - off - 1; > > > > > > if (new > old) > > > d2 = new - old; > > > else > > > d2 = new + vq->num - old; > > > > > > return d1 < d2; > > > } > > > > > > Thanks > > > >
WARNING: multiple messages have this Message-ID (diff)
From: Tiwei Bie <tiwei.bie@intel.com> To: Jason Wang <jasowang@redhat.com> Cc: "Michael S. Tsirkin" <mst@redhat.com>, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, wexu@redhat.com Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring Date: Tue, 8 May 2018 17:16:28 +0800 [thread overview] Message-ID: <20180508091628.d7jzpopqopq4abhy@debian> (raw) In-Reply-To: <34f2c690-7cb2-f9ea-2ce9-40f4ccb594c9@redhat.com> On Tue, May 08, 2018 at 03:16:53PM +0800, Jason Wang wrote: > On 2018年05月08日 14:44, Tiwei Bie wrote: > > On Tue, May 08, 2018 at 01:40:40PM +0800, Jason Wang wrote: > > > On 2018年05月08日 11:05, Jason Wang wrote: > > > > > Because in virtqueue_enable_cb_delayed(), we may set an > > > > > event_off which is bigger than new and both of them have > > > > > wrapped. And in this case, although new is smaller than > > > > > event_off (i.e. the third param -- old), new shouldn't > > > > > add vq->num, and actually we are expecting a very big > > > > > idx diff. > > > > Yes, so to calculate distance correctly between event and new, we just > > > > need to compare the warp counter and return false if it doesn't match > > > > without the need to try to add vq.num here. > > > > > > > > Thanks > > > Sorry, looks like the following should work, we need add vq.num if > > > used_wrap_counter does not match: > > > > > > static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq, > > > __u16 off_wrap, __u16 new, > > > __u16 old) > > > { > > > bool wrap = off_wrap >> 15; > > > int off = off_wrap & ~(1 << 15); > > > __u16 d1, d2; > > > > > > if (wrap != vq->used_wrap_counter) > > > d1 = new + vq->num - off - 1; > > Just to draw your attention (maybe you have already > > noticed this). > > I miss this, thanks! > > > > > In this case (i.e. wrap != vq->used_wrap_counter), > > it's also possible that (off < new) is true. Because, > > > > when virtqueue_enable_cb_delayed_packed() is used, > > `off` is calculated in driver in a way like this: > > > > off = vq->last_used_idx + bufs; > > if (off >= vq->vring_packed.num) { > > off -= vq->vring_packed.num; > > wrap_counter ^= 1; > > } > > > > And when `new` (in vhost) is close to vq->num. The > > vq->last_used_idx + bufs (in driver) can be bigger > > than vq->vring_packed.num, and: > > > > 1. `off` will wrap; > > 2. wrap counters won't match; > > 3. off < new; > > > > And d1 (i.e. new + vq->num - off - 1) will be a value > > bigger than vq->num. I'm okay with this, although it's > > a bit weird. > > > So I'm considering something more compact by reusing vring_need_event() by > pretending a larger queue size and adding vq->num back when necessary: > > static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq, > __u16 off_wrap, __u16 new, > __u16 old) > { > bool wrap = vq->used_wrap_counter; If the wrap counter is obtained from the vq, I think `new` should also be obtained from the vq. Or the wrap counter should be carried in `new`. > int off = off_wrap & ~(1 << 15); > __u16 d1, d2; > > if (new < old) { > new += vq->num; > wrap ^= 1; > } > > if (wrap != off_wrap >> 15) > off += vq->num; When `new` and `old` wraps, and `off` doesn't wrap, wrap != (off_wrap >> 15) will be true. In this case, `off` is bigger than `new`, and what we should do is `off -= vq->num` instead of `off += vq->num`. Best regards, Tiwei Bie > > return vring_need_event(off, new, old); > } > > > > > > Best regards, > > Tiwei Bie > > > > > else > > > d1 = new - off - 1; > > > > > > if (new > old) > > > d2 = new - old; > > > else > > > d2 = new + vq->num - old; > > > > > > return d1 < d2; > > > } > > > > > > Thanks > > > > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
next prev parent reply other threads:[~2018-05-08 9:16 UTC|newest] Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-04-25 5:15 [RFC v3 0/5] virtio: support packed ring Tiwei Bie 2018-04-25 5:15 ` [RFC v3 1/5] virtio: add packed ring definitions Tiwei Bie 2018-04-25 5:15 ` Tiwei Bie 2018-04-25 5:15 ` [RFC v3 2/5] virtio_ring: support creating packed ring Tiwei Bie 2018-04-25 5:15 ` Tiwei Bie 2018-04-25 5:15 ` [RFC v3 3/5] virtio_ring: add packed ring support Tiwei Bie 2018-05-10 7:32 ` Jason Wang 2018-05-10 7:32 ` Jason Wang 2018-05-10 7:34 ` Jason Wang 2018-05-10 7:34 ` Jason Wang 2018-05-10 8:56 ` Tiwei Bie 2018-05-10 8:56 ` Tiwei Bie 2018-05-10 9:49 ` Jason Wang 2018-05-10 10:50 ` Tiwei Bie 2018-05-10 10:50 ` Tiwei Bie 2018-05-10 9:49 ` Jason Wang 2018-04-25 5:15 ` Tiwei Bie 2018-04-25 5:15 ` [RFC v3 4/5] virtio_ring: add event idx support in packed ring Tiwei Bie 2018-05-02 2:51 ` Jason Wang 2018-05-02 2:51 ` Jason Wang 2018-05-02 7:28 ` Tiwei Bie 2018-05-02 13:51 ` Michael S. Tsirkin 2018-05-02 15:12 ` Tiwei Bie 2018-05-02 15:12 ` Tiwei Bie 2018-05-02 15:42 ` Michael S. Tsirkin 2018-05-02 15:42 ` Michael S. Tsirkin 2018-05-03 1:11 ` Tiwei Bie 2018-05-03 1:11 ` Tiwei Bie 2018-05-03 1:44 ` Michael S. Tsirkin 2018-05-03 1:44 ` Michael S. Tsirkin 2018-05-03 2:09 ` Tiwei Bie 2018-05-03 2:09 ` Tiwei Bie 2018-05-03 7:25 ` Jason Wang 2018-05-03 7:25 ` Jason Wang 2018-05-03 13:54 ` Tiwei Bie 2018-05-03 13:54 ` Tiwei Bie 2018-05-08 3:05 ` Jason Wang 2018-05-08 3:05 ` Jason Wang 2018-05-08 5:40 ` Jason Wang 2018-05-08 6:44 ` Tiwei Bie 2018-05-08 7:16 ` Jason Wang 2018-05-08 7:16 ` Jason Wang 2018-05-08 9:16 ` Tiwei Bie [this message] 2018-05-08 9:16 ` Tiwei Bie 2018-05-08 9:34 ` Jason Wang 2018-05-08 9:34 ` Jason Wang 2018-05-08 9:44 ` Tiwei Bie 2018-05-08 9:44 ` Tiwei Bie 2018-05-09 3:43 ` Jason Wang 2018-05-09 3:43 ` Jason Wang 2018-05-08 6:44 ` Tiwei Bie 2018-05-08 5:40 ` Jason Wang 2018-05-02 13:51 ` Michael S. Tsirkin 2018-05-02 7:28 ` Tiwei Bie 2018-05-16 5:01 ` Jason Wang 2018-05-16 5:55 ` Tiwei Bie 2018-05-16 5:55 ` Tiwei Bie 2018-05-16 5:01 ` Jason Wang 2018-04-25 5:15 ` Tiwei Bie 2018-04-25 5:15 ` [RFC v3 5/5] virtio_ring: enable " Tiwei Bie 2018-04-25 5:15 ` Tiwei Bie 2018-04-27 3:56 ` [RFC v3 0/5] virtio: support " Jason Wang 2018-04-27 4:18 ` Michael S. Tsirkin 2018-04-27 6:17 ` Jason Wang 2018-04-27 9:12 ` Tiwei Bie 2018-04-27 9:12 ` Tiwei Bie 2018-04-28 2:45 ` Jason Wang 2018-04-28 2:45 ` Jason Wang 2018-04-27 6:17 ` Jason Wang 2018-04-27 4:18 ` Michael S. Tsirkin 2018-04-27 3:56 ` Jason Wang 2018-05-02 3:49 ` Jason Wang 2018-05-02 3:49 ` Jason Wang
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