Michael S. Tsirkin
2022-Feb-03 11:32 UTC
[PATCH v2 3/9] [RFC] virtio_ring: Embed a wrap counter in opaque poll index value
On Thu, Feb 03, 2022 at 10:51:19AM +0000, Cristian Marussi wrote:> 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 at redhat.com> > > > Cc: Igor Skalkin <igor.skalkin at opensynergy.com> > > > Cc: Peter Hilber <peter.hilber at opensynergy.com> > > > Cc: virtualization at lists.linux-foundation.org > > > Signed-off-by: Cristian Marussi <cristian.marussi at 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 ?The later.> > 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)can be either or both, virtio_test/vhost is a bit easier to use.> > > > > > > > > @@ -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