From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932251AbeEHJng (ORCPT ); Tue, 8 May 2018 05:43:36 -0400 Received: from mga02.intel.com ([134.134.136.20]:28622 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754344AbeEHJnf (ORCPT ); Tue, 8 May 2018 05:43:35 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.49,377,1520924400"; d="scan'208";a="52461048" Date: Tue, 8 May 2018 17:44:06 +0800 From: Tiwei Bie To: Jason Wang Cc: "Michael S. Tsirkin" , 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 Message-ID: <20180508094406.qjlaism3hqy4hvjd@debian> References: <20180503044218-mutt-send-email-mst@kernel.org> <20180503020949.5u3qz32gsk33z6vk@debian> <9f0b4e37-63ff-42f9-f2e6-3747a19a0206@redhat.com> <20180503135430.lbtvn4p4lyu3ksqo@debian> <12ede490-f674-2b89-d639-266b5fe15466@redhat.com> <20180508064409.kcn6amhsxu7nkuuc@debian> <34f2c690-7cb2-f9ea-2ce9-40f4ccb594c9@redhat.com> <20180508091628.d7jzpopqopq4abhy@debian> <122277c6-d103-e1f6-d695-4d64e6934a51@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <122277c6-d103-e1f6-d695-4d64e6934a51@redhat.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 08, 2018 at 05:34:40PM +0800, Jason Wang wrote: > On 2018年05月08日 17:16, Tiwei Bie wrote: > > 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`. > > If I understand this correctly, if we track old correctly, it won't happen > if guest driver behave correctly. That means it should only happen for a > buggy driver (e.g trying to move off_wrap back). If vhost is faster than virtio driver, I guess above case may happen. The `old` and `new` will be updated each time we want to notify the driver. If the driver is slower, `old` and `new` in vhost may wrap before the `off` which is set by driver wraps. Best regards, Tiwei Bie > > Thanks > > > > > 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 > > > > > >