Re: [PATCH v2 3/9] [RFC] virtio_ring: Embed a wrap counter in opaque poll index value

Cristian Marussi
2022-02-03T10:51:25Z

On Tue, Feb 01, 2022 at 01:27:38PM -0500, Michael S. Tsirkin wrote:
> Looks correct, thanks. Some minor comments below:
>

Hi Michael,

thanks for the feedback.

> On Tue, Feb 01, 2022 at 05:15:55PM +0000, Cristian Marussi wrote:
> > Exported API virtqueue_poll() can be used to support polling mode operation
> > on top of virtio layer if needed; currently the parameter last_used_idx is
> > the opaque value that needs to be passed to the virtqueue_poll() function
> > to check if there are new pending used buffers in the queue: such opaque
> > value would have been previously obtained by a call to the API function
> > virtqueue_enable_cb_prepare().
> >
> > Since such opaque value is indeed containing simply a snapshot in time of
> > the internal
>
> to add: 16 bit
>
> > last_used_index (roughly), it is possible that,
>
> to add here:
>
> if another thread calls virtqueue_add_*()
> at the same time (which existing drivers don't do,
> but does not seem to be documented as prohibited anywhere), and
>
> > if exactly
> > 2**16 buffers are marked as used between two successive calls to
> > virtqueue_poll(), the caller is fooled into thinking that nothing is
> > pending (ABA problem).
> > Keep a full fledged internal wraps counter
>
> s/full fledged/a 16 bit/
>
> since I don't see why is a 16 bit counter full but not e.g. a 32 bit one
>
.. :D I wanted to stress the fact that this being a 16bits counter has a
higher rollover than a 1-bit one wrap_counter already used...but indeed
all are just counters at the end, it's justthe wrapround that changes...

I'll fix.

> > per virtqueue and embed it into
> > the upper 16bits of the returned opaque value, so that the above scenario
> > can be detected transparently by virtqueue_poll(): this way each single
> > possible last_used_idx value is really belonging to a different wrap.
>
> Just to add here: the ABA problem can in theory still happen but
> now that's after 2^32 requests, which seems sufficient in practice.
>

Sure, I'll fix the commit message as above advised.

> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
> > Cc: Peter Hilber <peter.hilber@opensynergy.com>
> > Cc: virtualization@lists.linux-foundation.org
> > Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> > ---
> > Still no perf data on this, I was wondering what exactly to measure in
> > term of perf metrics to evaluate the impact of the rolling vq->wraps
> > counter.
> > ---
> > drivers/virtio/virtio_ring.c | 51 +++++++++++++++++++++++++++++++++---
> > 1 file changed, 47 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 00f64f2f8b72..613ec0503509 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -12,6 +12,8 @@
> > #include <linux/hrtimer.h>
> > #include <linux/dma-mapping.h>
> > #include <linux/spinlock.h>
> > +#include <linux/bits.h>
> > +#include <linux/bitfield.h>
> > #include <xen/xen.h>
> >
> > static bool force_used_validation = false;
> > @@ -69,6 +71,17 @@ module_param(force_used_validation, bool, 0444);
> > #define LAST_ADD_TIME_INVALID(vq)
> > #endif
> >
> > +#define VRING_IDX_MASK GENMASK(15, 0)
> > +#define VRING_GET_IDX(opaque) \
> > + ((u16)FIELD_GET(VRING_IDX_MASK, (opaque)))
> > +
> > +#define VRING_WRAPS_MASK GENMASK(31, 16)
> > +#define VRING_GET_WRAPS(opaque) \
> > + ((u16)FIELD_GET(VRING_WRAPS_MASK, (opaque)))
> > +
> > +#define VRING_BUILD_OPAQUE(idx, wraps) \
> > + (FIELD_PREP(VRING_WRAPS_MASK, (wraps)) | ((idx) & VRING_IDX_MASK))
> > +
>
> Maybe prefix with VRING_POLL_ since that is the only user.
>

I'll do.

>
> > struct vring_desc_state_split {
> > void *data; /* Data for callback. */
> > struct vring_desc *indir_desc; /* Indirect descriptor, if any. */
> > @@ -117,6 +130,8 @@ struct vring_virtqueue {
> > /* Last used index we've seen. */
> > u16 last_used_idx;
> >
> > + u16 wraps;
> > +
> > /* Hint for event idx: already triggered no need to disable. */
> > bool event_triggered;
> >
> > @@ -806,6 +821,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> > ret = vq->split.desc_state[i].data;
> > detach_buf_split(vq, i, ctx);
> > vq->last_used_idx++;
> > + if (unlikely(!vq->last_used_idx))
> > + vq->wraps++;
> > /* 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. */
>
> So most drivers don't call virtqueue_poll.
> Concerned about the overhead here: another option is
> with a flag that will have to be set whenever a driver
> wants to use virtqueue_poll.

Do you mean a compile time flag/Kconfig to just remove the possible
overhead instructions as a whole when not needed by the driver ?

Or do you mean at runtime since checking the flag evry time should be
less costly than checking the wrpas each time AND counting when it
happens ?

> Could you pls do a quick perf test e.g. using tools/virtio/
> to see what's faster?

Yes I'll do, thanks for the hint, I have some compilation issues in
tools/virtio due to my additions (missing mirrored hehaders) or to some
recently added stuff (missing drv_to_virtio & friends for
suppressed_used_validation thing)...anyway I fixed those now and I'll
post related tools/virtio patches with next iteration.

Anyway, do you mean perf data about vringh_test and virtio_test/vhost
right ? (ringtest/ excluded 'cause does not use any API is just
prototyping)

>
>
>
> > @@ -1508,6 +1525,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> > if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
> > vq->last_used_idx -= vq->packed.vring.num;
> > vq->packed.used_wrap_counter ^= 1;
> > + vq->wraps++;
> > }
> >
> > /*
> > @@ -1744,6 +1762,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > vq->weak_barriers = weak_barriers;
> > vq->broken = false;
> > vq->last_used_idx = 0;
> > + vq->wraps = 0;
> > vq->event_triggered = false;
> > vq->num_added = 0;
> > vq->packed_ring = true;
> > @@ -2092,13 +2111,17 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
> > */
> > unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> > {
> > + unsigned int last_used_idx;
> > struct vring_virtqueue *vq = to_vvq(_vq);
> >
> > if (vq->event_triggered)
> > vq->event_triggered = false;
> >
> > - return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(_vq) :
> > - virtqueue_enable_cb_prepare_split(_vq);
> > + last_used_idx = vq->packed_ring ?
> > + virtqueue_enable_cb_prepare_packed(_vq) :
> > + virtqueue_enable_cb_prepare_split(_vq);
> > +
> > + return VRING_BUILD_OPAQUE(last_used_idx, vq->wraps);
> > }
> > EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> >
> > @@ -2107,6 +2130,21 @@ EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> > * @_vq: the struct virtqueue we're talking about.
> > * @last_used_idx: virtqueue state (from call to virtqueue_enable_cb_prepare).
> > *
> > + * The provided last_used_idx, as returned by virtqueue_enable_cb_prepare(),
> > + * is an opaque value representing the queue state and it is built as follows:
> > + *
> > + * ---------------------------------------------------------
> > + * | vq->wraps | vq->last_used_idx |
> > + * 31------------------------------------------------------0
> > + *
> > + * The MSB 16bits embedding the wraps counter for the underlying virtqueue
> > + * is stripped out here before reaching into the lower layer helpers.
> > + *
> > + * This structure of the opaque value mitigates the scenario in which, when
> > + * exactly 2**16 messages are marked as used between two successive calls to
> > + * virtqueue_poll(), the caller is fooled into thinking nothing new has arrived
> > + * since the pure last_used_idx is exactly the same.
> > + *
>
> Do you want to move this comment to where the macros implementing it
> are?
>

Sure, I'll do.

Thanks,
Cristian