Re: [PATCH 5 of 5] virtio: expose added descriptors immediately
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2011-11-21 11:55:51
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kvm, lkml
On Mon, Nov 21, 2011 at 12:18:45PM +1030, Rusty Russell wrote:
On Wed, 16 Nov 2011 09:18:38 +0200, "Michael S. Tsirkin" [off-list ref] wrote:quoted
My unlocked kick patches will trip this warning: they make virtio-net do add + get without kick.Heh, it's a good sign if they do, since that means you're running really well :)
They don't in fact, in my testing :(. But I think they can with luck.
quoted
I think block with unlocked kick can trip it too: add, lock is dropped and then an interrupt can get. We also don't need a kick each num - each 2^15 is enough. Why don't we do this at start of add_buf: if (vq->num_added >= 0x7fff) return -ENOSPC;The warning was there in case a driver is never doing a kick, and getting away with it (mostly) because the device is polling. Let's not penalize good drivers to catch bad ones. How about we do this properly, like so:
Absolutely. But I think we also need to handle num_added overflow of a 15 bit counter, no? Otherwise the vring_need_event logic might give us false negatives .... I'm guessing we can just assume we need a kick in that case.
quoted hunk ↗ jump to hunk
From: Rusty Russell <redacted> Subject: virtio: add debugging if driver doesn't kick. Under the existing #ifdef DEBUG, check that they don't have more than 1/10 of a second between an add_buf() and a virtqueue_notify()/virtqueue_kick_prepare() call. We could get false positives on a really busy system, but good for development. Signed-off-by: Rusty Russell <redacted> --- drivers/virtio/virtio_ring.c | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+)diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c@@ -22,6 +22,7 @@ #include <linux/device.h> #include <linux/slab.h> #include <linux/module.h> +#include <linux/hrtimer.h> /* virtio guest is communicating with a virtual "device" that actually runs on * a host processor. Memory barriers are used to control SMP effects. */@@ -102,6 +103,10 @@ struct vring_virtqueue #ifdef DEBUG /* They're supposed to lock for us. */ unsigned int in_use; + + /* Figure out if their kicks are too delayed. */ + bool last_add_time_valid; + ktime_t last_add_time; #endif /* Tokens for callbacks. */@@ -192,6 +197,19 @@ int virtqueue_add_buf(struct virtqueue * BUG_ON(data == NULL); +#ifdef DEBUG + { + ktime_t now = ktime_get(); + + /* No kick or get, with .1 second between? Warn. */ + if (vq->last_add_time_valid) + WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time)) + > 100); + vq->last_add_time = now; + vq->last_add_time_valid = true; + } +#endif + /* If the host supports indirect descriptor tables, and we have multiple * buffers, then go indirect. FIXME: tune this threshold */ if (vq->indirect && (out + in) > 1 && vq->num_free) {@@ -291,6 +309,14 @@ bool virtqueue_kick_prepare(struct virtq new = vq->vring.avail->idx; vq->num_added = 0; +#ifdef DEBUG + if (vq->last_add_time_valid) { + WARN_ON(ktime_to_ms(ktime_sub(ktime_get(), + vq->last_add_time)) > 100); + } + vq->last_add_time_valid = false; +#endif + if (vq->event) { needs_kick = vring_need_event(vring_avail_event(&vq->vring), new, old);@@ -428,6 +454,10 @@ void *virtqueue_get_buf(struct virtqueue virtio_mb(); } +#ifdef DEBUG + vq->last_add_time_valid = false; +#endif + END_USE(vq); return ret; }@@ -611,6 +641,7 @@ struct virtqueue *vring_new_virtqueue(un list_add_tail(&vq->vq.list, &vdev->vqs); #ifdef DEBUG vq->in_use = false; + vq->last_add_time_valid = false; #endif vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC);