Messages in this thread | | | Date | Wed, 27 Mar 2024 14:08:47 +1000 | Subject | Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty() | From | Gavin Shan <> |
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On 3/27/24 12:44, Jason Wang wrote: > On Wed, Mar 27, 2024 at 10:34 AM Jason Wang <jasowang@redhat.com> wrote: >> On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@redhat.com> wrote: >>> >>> A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by >>> Will Deacon <will@kernel.org>. 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@kernel.org> # v4.11+ >>> Reported-by: Yihuang Yu <yihyu@redhat.com> >>> Signed-off-by: Gavin Shan <gshan@redhat.com> >>> --- >>> 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.
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
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