Messages in this thread | | | Date | Wed, 27 Mar 2024 17:35:32 +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 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@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. >
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
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