Messages in this thread | | | Date | Fri, 19 Jan 2018 21:49:38 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH 1/2] virtio: allow to detach a buffer from the virtqueue |
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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 <groug@kaod.org>
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); >
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