On 3/27/24 12:41, Jason Wang wrote:
On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@xxxxxxxxxx> wrote:A smp_rmb() has been missed in vhost_enable_notify(), inspired 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_enable_notify(). Note that it should be safe until vq->avail_idx is changed by commit d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()"). Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()") Cc: <stable@xxxxxxxxxx> # v5.18+ 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 00445ab172b3..58f9d6a435f0 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2847,9 +2847,18 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) &vq->avail->idx, 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. + */ + smp_rmb(); + return true; + } - return vq->avail_idx != vq->last_avail_idx; + return false;So we only care about the case when vhost_enable_notify() returns true. In that case, I think you want to order with vhost_get_vq_desc(): last_avail_idx = vq->last_avail_idx; if (vq->avail_idx == vq->last_avail_idx) { /* false */ } vhost_get_avail_head(vq, &ring_head, last_avail_idx) Assuming I understand the patch correctly. Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
Jason, thanks for your review and comments. Your understanding is exactly what I understood.
} EXPORT_SYMBOL_GPL(vhost_enable_notify);
Thanks, Gavin
