Re: [PATCH] virtio_ring: Fix the stale index in available ring
From: Gavin Shan
Date: Tue Mar 19 2024 - 04:12:01 EST
On 3/19/24 17:09, Michael S. Tsirkin wrote:
On Tue, Mar 19, 2024 at 04:49:50PM +1000, Gavin Shan wrote:
On 3/19/24 16:43, Michael S. Tsirkin wrote:
On Tue, Mar 19, 2024 at 04:38:49PM +1000, Gavin Shan wrote:
On 3/19/24 16:09, Michael S. Tsirkin wrote:
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 49299b1f9ec7..7d852811c912 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -687,9 +687,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
- /* Descriptors and available array need to be set before we expose the
- * new available array entries. */
- virtio_wmb(vq->weak_barriers);
+ /*
+ * Descriptors and available array need to be set before we expose
+ * the new available array entries. virtio_wmb() should be enough
+ * to ensuere the order theoretically. However, a stronger barrier
+ * is needed by ARM64. Otherwise, the stale data can be observed
+ * by the host (vhost). A stronger barrier should work for other
+ * architectures, but performance loss is expected.
+ */
+ virtio_mb(false);
vq->split.avail_idx_shadow++;
vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->split.avail_idx_shadow);
Replacing a DMB with a DSB is _very_ unlikely to be the correct solution
here, especially when ordering accesses to coherent memory.
In practice, either the larger timing different from the DSB or the fact
that you're going from a Store->Store barrier to a full barrier is what
makes things "work" for you. Have you tried, for example, a DMB SY
(e.g. via __smb_mb()).
We definitely shouldn't take changes like this without a proper
explanation of what is going on.
Thanks for your comments, Will.
Yes, DMB should work for us. However, it seems this instruction has issues on
NVidia's grace-hopper. It's hard for me to understand how DMB and DSB works
from hardware level. I agree it's not the solution to replace DMB with DSB
before we fully understand the root cause.
I tried the possible replacement like below. __smp_mb() can avoid the issue like
__mb() does. __ndelay(10) can avoid the issue, but __ndelay(9) doesn't.
static inline int virtqueue_add_split(struct virtqueue *_vq, ...)
{
:
/* Put entry in available array (but don't update avail->idx until they
* do sync). */
avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
/* Descriptors and available array need to be set before we expose the
* new available array entries. */
// Broken: virtio_wmb(vq->weak_barriers);
// Broken: __dma_mb();
// Work: __mb();
// Work: __smp_mb();
// Work: __ndelay(100);
// Work: __ndelay(10);
// Broken: __ndelay(9);
vq->split.avail_idx_shadow++;
vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->split.avail_idx_shadow);
What if you stick __ndelay here?
/* Put entry in available array (but don't update avail->idx until they
* do sync). */
avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
/* Descriptors and available array need to be set before we expose the
* new available array entries. */
virtio_wmb(vq->weak_barriers);
vq->split.avail_idx_shadow++;
vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->split.avail_idx_shadow);
/* Try __ndelay(x) here as Michael suggested
*
* Work: __ndelay(200); possiblly make it hard to reproduce
* Broken: __ndelay(100);
* Broken: __ndelay(20);
* Broken: __ndelay(10);
*/
__ndelay(200);
So we see that just changing the timing masks the race.
What are you using on the host side? vhost or qemu?
__ndelay(200) may make the issue harder to be reproduce as I understand.
More delays here will give vhost relief, reducing the race.
The issue is only reproducible when vhost is turned on. Otherwise, we
aren't able to hit the issue.
-netdev tap,id=vnet0,vhost=true,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown \
-device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0
Given it's vhost, it's also possible that the issue is host side.
I wonder what happens if we stick a delay or a stronger barrier
in vhost.c - either here:
/* Make sure buffer is written before we update index. */
smp_wmb();
or here:
/* Only get avail ring entries after they have been
* exposed by guest.
*/
smp_rmb();
?
It's possible. However, I still can hit the issue after both of them are
replaed with '__mb()'. So the issue seems on the guest side, where the
written data isn't observed in time by the CPU on far end (for vhost worker).
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 045f666b4f12..327b68d21b02 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2529,7 +2529,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
/* Only get avail ring entries after they have been
* exposed by guest.
*/
- smp_rmb();
+ // smp_rmb();
+ __mb();
}
/* Grab the next descriptor number they're advertising, and increment
@@ -2703,7 +2704,9 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
r = __vhost_add_used_n(vq, heads, count);
/* Make sure buffer is written before we update index. */
- smp_wmb();
+ // smp_wmb();
+ __mb();
+
From the guest:
[ 14.102608] virtio_net virtio0: output.0:id 80 is not a head!
vq->num_added++;
pr_debug("Added buffer head %i to %p\n", head, vq);
END_USE(vq);
:
}
I also tried to measure the consumed time for various barrier-relative instructions using
ktime_get_ns() which should have consumed most of the time. __smb_mb() is slower than
__smp_wmb() but faster than __mb()
Instruction Range of used time in ns
----------------------------------------------
__smp_wmb() [32 1128032]
__smp_mb() [32 1160096]
__mb() [32 1162496]
Thanks,
Gavin