From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933709AbeEIDnY (ORCPT ); Tue, 8 May 2018 23:43:24 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:59368 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932115AbeEIDnW (ORCPT ); Tue, 8 May 2018 23:43:22 -0400 Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring To: Tiwei Bie Cc: "Michael S. Tsirkin" , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, wexu@redhat.com, jfreimann@redhat.com 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> <20180508094406.qjlaism3hqy4hvjd@debian> From: Jason Wang Message-ID: Date: Wed, 9 May 2018 11:43:15 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180508094406.qjlaism3hqy4hvjd@debian> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018年05月08日 17:44, Tiwei Bie wrote: > 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 > Oh, right. But the code still work (in this case new - event_idx - 1 will underflow). (And I admit it still looks ugly). Thanks