From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751901AbeEBPMS (ORCPT ); Wed, 2 May 2018 11:12:18 -0400 Received: from mga02.intel.com ([134.134.136.20]:55411 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751630AbeEBPMM (ORCPT ); Wed, 2 May 2018 11:12:12 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.49,354,1520924400"; d="scan'208";a="225171674" Date: Wed, 2 May 2018 23:12:55 +0800 From: Tiwei Bie To: "Michael S. Tsirkin" Cc: Jason Wang , 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: <20180502151255.h3x6rhszxa3euinl@debian> References: <20180425051550.24342-1-tiwei.bie@intel.com> <20180425051550.24342-5-tiwei.bie@intel.com> <34781052-df9f-e505-cd3f-08e460b34dcc@redhat.com> <20180502072819.mf5l3dypk6dwx2s7@debian> <20180502164828-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180502164828-mutt-send-email-mst@kernel.org> 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 Wed, May 02, 2018 at 04:51:01PM +0300, Michael S. Tsirkin wrote: > On Wed, May 02, 2018 at 03:28:19PM +0800, Tiwei Bie wrote: > > On Wed, May 02, 2018 at 10:51:06AM +0800, Jason Wang wrote: > > > On 2018年04月25日 13:15, Tiwei Bie wrote: > > > > This commit introduces the event idx support in packed > > > > ring. This feature is temporarily disabled, because the > > > > implementation in this patch may not work as expected, > > > > and some further discussions on the implementation are > > > > needed, e.g. do we have to check the wrap counter when > > > > checking whether a kick is needed? > > > > > > > > Signed-off-by: Tiwei Bie > > > > --- > > > > drivers/virtio/virtio_ring.c | 53 ++++++++++++++++++++++++++++++++++++++++---- > > > > 1 file changed, 49 insertions(+), 4 deletions(-) > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > index 0181e93897be..b1039c2985b9 100644 > > > > --- a/drivers/virtio/virtio_ring.c > > > > +++ b/drivers/virtio/virtio_ring.c > > > > @@ -986,7 +986,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq, > > > > static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq) > > > > { > > > > struct vring_virtqueue *vq = to_vvq(_vq); > > > > - u16 flags; > > > > + u16 new, old, off_wrap, flags; > > > > bool needs_kick; > > > > u32 snapshot; > > > > @@ -995,7 +995,12 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq) > > > > * suppressions. */ > > > > virtio_mb(vq->weak_barriers); > > > > + old = vq->next_avail_idx - vq->num_added; > > > > + new = vq->next_avail_idx; > > > > + vq->num_added = 0; > > > > + > > > > snapshot = *(u32 *)vq->vring_packed.device; > > > > + off_wrap = virtio16_to_cpu(_vq->vdev, snapshot & 0xffff); > > > > flags = cpu_to_virtio16(_vq->vdev, snapshot >> 16) & 0x3; > > > > #ifdef DEBUG > > > > @@ -1006,7 +1011,10 @@ static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq) > > > > vq->last_add_time_valid = false; > > > > #endif > > > > - needs_kick = (flags != VRING_EVENT_F_DISABLE); > > > > + if (flags == VRING_EVENT_F_DESC) > > > > + needs_kick = vring_need_event(off_wrap & ~(1<<15), new, old); > > > > > > I wonder whether or not the math is correct. Both new and event are in the > > > unit of descriptor ring size, but old looks not. > > > > What vring_need_event() cares is the distance between > > `new` and `old`, i.e. vq->num_added. So I think there > > is nothing wrong with `old`. But the calculation of the > > distance between `new` and `event_idx` isn't right when > > `new` wraps. How do you think about the below code: > > > > wrap_counter = off_wrap >> 15; > > event_idx = off_wrap & ~(1<<15); > > if (wrap_counter != vq->wrap_counter) > > event_idx -= vq->vring_packed.num; > > > > needs_kick = vring_need_event(event_idx, new, old); > > I suspect this hack won't work for non power of 2 ring. Above code doesn't require the ring size to be a power of 2. For (__u16)(new_idx - old), what we want to get is vq->num_added. old = vq->next_avail_idx - vq->num_added; new = vq->next_avail_idx; When vq->next_avail_idx >= vq->num_added, it's obvious that, (__u16)(new_idx - old) is vq->num_added. And when vq->next_avail_idx < vq->num_added, new will be smaller than old (old will be a big unsigned number), but (__u16)(new_idx - old) is still vq->num_added. For (__u16)(new_idx - event_idx - 1), when new wraps and event_idx doesn't wrap, the most straightforward way to calculate it is: (new + vq->vring_packed.num) - event_idx - 1. But we can also calculate it in this way: event_idx -= vq->vring_packed.num; (event_idx will be a big unsigned number) Then (__u16)(new_idx - event_idx - 1) will be the value we want. Best regards, Tiwei Bie > > > > Best regards, > > Tiwei Bie > > > > > > > > > > Thanks > > > > > > > + else > > > > + needs_kick = (flags != VRING_EVENT_F_DISABLE); > > > > END_USE(vq); > > > > return needs_kick; > > > > } > > > > @@ -1116,6 +1124,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, > > > > if (vq->last_used_idx >= vq->vring_packed.num) > > > > vq->last_used_idx -= vq->vring_packed.num; > > > > + /* If we expect an interrupt for the next entry, tell host > > > > + * by writing event index and flush out the write before > > > > + * the read in the next get_buf call. */ > > > > + if (vq->event_flags_shadow == VRING_EVENT_F_DESC) > > > > + virtio_store_mb(vq->weak_barriers, > > > > + &vq->vring_packed.driver->off_wrap, > > > > + cpu_to_virtio16(_vq->vdev, vq->last_used_idx | > > > > + (vq->wrap_counter << 15))); > > > > + > > > > #ifdef DEBUG > > > > vq->last_add_time_valid = false; > > > > #endif > > > > @@ -1143,10 +1160,17 @@ static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq) > > > > /* We optimistically turn back on interrupts, then check if there was > > > > * more to do. */ > > > > + /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to > > > > + * either clear the flags bit or point the event index at the next > > > > + * entry. Always update the event index to keep code simple. */ > > > > + > > > > + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev, > > > > + vq->last_used_idx | (vq->wrap_counter << 15)); > > > > if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) { > > > > virtio_wmb(vq->weak_barriers); > > > > - vq->event_flags_shadow = VRING_EVENT_F_ENABLE; > > > > + vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC : > > > > + VRING_EVENT_F_ENABLE; > > > > vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev, > > > > vq->event_flags_shadow); > > > > } > > > > @@ -1172,15 +1196,34 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned last_used_idx) > > > > static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq) > > > > { > > > > struct vring_virtqueue *vq = to_vvq(_vq); > > > > + u16 bufs, used_idx, wrap_counter; > > > > START_USE(vq); > > > > /* We optimistically turn back on interrupts, then check if there was > > > > * more to do. */ > > > > + /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to > > > > + * either clear the flags bit or point the event index at the next > > > > + * entry. Always update the event index to keep code simple. */ > > > > + > > > > + /* TODO: tune this threshold */ > > > > + bufs = (u16)(vq->next_avail_idx - vq->last_used_idx) * 3 / 4; > > > > + > > > > + used_idx = vq->last_used_idx + bufs; > > > > + wrap_counter = vq->wrap_counter; > > > > + > > > > + if (used_idx >= vq->vring_packed.num) { > > > > + used_idx -= vq->vring_packed.num; > > > > + wrap_counter ^= 1; > > > > + } > > > > + > > > > + vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev, > > > > + used_idx | (wrap_counter << 15)); > > > > if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) { > > > > virtio_wmb(vq->weak_barriers); > > > > - vq->event_flags_shadow = VRING_EVENT_F_ENABLE; > > > > + vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC : > > > > + VRING_EVENT_F_ENABLE; > > > > vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev, > > > > vq->event_flags_shadow); > > > > } > > > > @@ -1822,8 +1865,10 @@ void vring_transport_features(struct virtio_device *vdev) > > > > switch (i) { > > > > case VIRTIO_RING_F_INDIRECT_DESC: > > > > break; > > > > +#if 0 > > > > case VIRTIO_RING_F_EVENT_IDX: > > > > break; > > > > +#endif > > > > case VIRTIO_F_VERSION_1: > > > > break; > > > > case VIRTIO_F_IOMMU_PLATFORM: > > >