Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty()
From: Gavin Shan
Date: Wed Mar 27 2024 - 03:35:54 EST
On 3/27/24 14:08, Gavin Shan wrote:
On 3/27/24 12:44, Jason Wang wrote:
On Wed, Mar 27, 2024 at 10:34 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@xxxxxxxxxx> wrote:
A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by
Will Deacon <will@xxxxxxxxxx>. Otherwise, it's not ensured the
available ring entries pushed by guest can be observed by vhost
in time, leading to stale available ring entries fetched by vhost
in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's
grace-hopper (ARM64) platform.
/home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
-accel kvm -machine virt,gic-version=host -cpu host \
-smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
-m 4096M,slots=16,maxmem=64G \
-object memory-backend-ram,id=mem0,size=4096M \
: \
-netdev tap,id=vnet0,vhost=true \
-device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0
:
guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
virtio_net virtio0: output.0:id 100 is not a head!
Add the missed smp_rmb() in vhost_vq_avail_empty(). Note that it
should be safe until vq->avail_idx is changed by commit 275bf960ac697
("vhost: better detection of available buffers").
Fixes: 275bf960ac697 ("vhost: better detection of available buffers")
Cc: <stable@xxxxxxxxxx> # v4.11+
Reported-by: Yihuang Yu <yihyu@xxxxxxxxxx>
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
---
drivers/vhost/vhost.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 045f666b4f12..00445ab172b3 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2799,9 +2799,18 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
r = vhost_get_avail_idx(vq, &avail_idx);
if (unlikely(r))
return false;
+
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+ if (vq->avail_idx != vq->last_avail_idx) {
+ /* Similar to what's done in vhost_get_vq_desc(), we need
+ * to ensure the available ring entries have been exposed
+ * by guest.
+ */
We need to be more verbose here. For example, which load needs to be
ordered with which load.
The rmb in vhost_get_vq_desc() is used to order the load of avail idx
and the load of head. It is paired with e.g virtio_wmb() in
virtqueue_add_split().
vhost_vq_avail_empty() are mostly used as a hint in
vhost_net_busy_poll() which is under the protection of the vq mutex.
An exception is the tx_can_batch(), but in that case it doesn't even
want to read the head.
Ok, if it is needed only in that path, maybe we can move the barriers there.
[cc Will Deacon]
Jason, appreciate for your review and comments. I think PATCH[1/2] is
the fix for the hypothesis, meaning PATCH[2/2] is the real fix. However,
it would be nice to fix all of them in one shoot. I will try with PATCH[2/2]
only to see if our issue will disappear or not. However, the issue still
exists if PATCH[2/2] is missed.
Jason, PATCH[2/2] is sufficient to fix our current issue. I tried with PATCH[2/2]
only and unable to hit the issue. However, PATCH[1/2] may be needed by other scenarios.
So it would be nice to fix them in one shoot.
Firstly, We were failing on the transmit queue and {tvq, rvq}->busyloop_timeout
== false if I remember correctly. So the added smp_rmb() in vhost_vq_avail_empty()
is only a concern to tx_can_batch(). A mutex isn't enough to ensure the order
for the available index and available ring entry (head). For example, vhost_vq_avail_empty()
called by tx_can_batch() can see next available index, but its corresponding
available ring entry (head) may not be seen by vhost yet if smp_rmb() is missed.
The next call to get_tx_bufs(), where the available ring entry (head) doesn't
arrived yet, leading to stale available ring entry (head) being fetched.
handle_tx_copy
get_tx_bufs // smp_rmb() won't be executed when vq->avail_idx != vq->last_avail_idx
tx_can_batch
vhost_vq_avail_empty // vq->avail_idx is updated from vq->avail->idx
The reason why I added smp_rmb() to vhost_vq_avail_empty() is because the function
is a exposed API, even it's only used by drivers/vhost/net.c at present. It means
the API has been broken internally. So it seems more appropriate to fix it up in
vhost_vq_avail_empty() so that the API's users needn't worry about the memory access
order.
+ smp_rmb();
+ return false;
+ }
- return vq->avail_idx == vq->last_avail_idx;
+ return true;
}
EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
Thanks,
Gavin