From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756241AbeASTtq (ORCPT ); Fri, 19 Jan 2018 14:49:46 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54726 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755982AbeASTtj (ORCPT ); Fri, 19 Jan 2018 14:49:39 -0500 Date: Fri, 19 Jan 2018 21:49:38 +0200 From: "Michael S. Tsirkin" To: Greg Kurz Cc: linux-kernel@vger.kernel.org, v9fs-developer@lists.sourceforge.net, Jason Wang , Al Viro Subject: Re: [PATCH 1/2] virtio: allow to detach a buffer from the virtqueue Message-ID: <20180119213953-mutt-send-email-mst@kernel.org> References: <151379198727.15509.2771563206512953068.stgit@bahia> <151379200283.15509.14269498487565699857.stgit@bahia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <151379200283.15509.14269498487565699857.stgit@bahia> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 20, 2017 at 06:46:42PM +0100, Greg Kurz wrote: > It is possible for a device to stop using buffers without pushing them > back to the driver. This is the case for example with the 9p virtio > device: if the driver flushes an in-flight request, the 9p specification > specification [*] mandates the server to "to purge the pending response". > The reply to the flush request indicates that the 9p server has stopped > using the buffers of the flushed in-flight request. But since the server > doesn't push back the associated buffers, they don't go back to the free > list. This leads the virtqueue to end up with a single slot to handle all > the dialog with the device, ie, serialize all I/Os. > > This patch hence gives the possibility for device specific code to > explicitly detach a given buffer from the used ring and put it back > to the free list. > > [*] http://man.cat-v.org/plan_9/5/flush > > Signed-off-by: Greg Kurz It would be better to just change the server to mark all flushed requests as used. Why isn't that an option? > --- > drivers/virtio/virtio_ring.c | 28 ++++++++++++++++++++++++++++ > include/linux/virtio.h | 1 + > 2 files changed, 29 insertions(+) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index eb30f3e09a47..886e9d054de3 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -936,6 +936,34 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq) > } > EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf); > > +/** > + * virtqueue_detach_buf - detach specific buffer > + * @vq: the struct virtqueue we're talking about. > + * > + * Returns NULL or the "data" token handed to virtqueue_add_*(). > + * This should only be used if the driver really knows the buffer > + * isn't needed anymore by the device. > + */ > +void *virtqueue_detach_buf(struct virtqueue *_vq, void *buf) > +{ > + struct vring_virtqueue *vq = to_vvq(_vq); > + unsigned int i; > + > + START_USE(vq); > + > + for (i = 0; i < vq->vring.num; i++) { > + if (vq->desc_state[i].data != buf) > + continue; > + detach_buf(vq, i, NULL); > + END_USE(vq); > + return buf; > + } > + > + END_USE(vq); > + return NULL; > +} > +EXPORT_SYMBOL_GPL(virtqueue_detach_buf); > + > irqreturn_t vring_interrupt(int irq, void *_vq) > { > struct vring_virtqueue *vq = to_vvq(_vq); Unfortunately the used index gets out of sync with the available index then. E.g. you are breaking the invariant that used == avail means ring empty. Any chance to preserve this invariant? > diff --git a/include/linux/virtio.h b/include/linux/virtio.h > index 988c7355bc22..850158518ce5 100644 > --- a/include/linux/virtio.h > +++ b/include/linux/virtio.h > @@ -80,6 +80,7 @@ bool virtqueue_poll(struct virtqueue *vq, unsigned); > bool virtqueue_enable_cb_delayed(struct virtqueue *vq); > > void *virtqueue_detach_unused_buf(struct virtqueue *vq); > +void *virtqueue_detach_buf(struct virtqueue *vq, void *buf); > > unsigned int virtqueue_get_vring_size(struct virtqueue *vq); >