linux-kernel - Re: [PATCH v3 2/3] vhost: Add smp_rmb() in vhost_enable

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Date: Thu, 28 Mar 2024 10:48:18 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Gavin Shan <gshan@...hat.com>
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	mst@...hat.com, jasowang@...hat.com, will@...nel.org, davem@...emloft.net, 
	stefanha@...hat.com, keirf@...gle.com, yihyu@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v3 2/3] vhost: Add smp_rmb() in vhost_enable_notify()

On Thu, Mar 28, 2024 at 10:21:48AM +1000, Gavin Shan wrote:
>A smp_rmb() has been missed in vhost_enable_notify(), inspired by
>Will. 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(). When it returns true,
>it means there's still pending tx buffers. Since it might read indices,
>so it still can bypass the smp_rmb() in vhost_get_vq_desc(). 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@...nel.org> # v5.18+
>Reported-by: Yihuang Yu <yihyu@...hat.com>
>Suggested-by: Will Deacon <will@...nel.org>
>Signed-off-by: Gavin Shan <gshan@...hat.com>
>Acked-by: Jason Wang <jasowang@...hat.com>
>---
> drivers/vhost/vhost.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)

Thanks for fixing this!

Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index 29df65b2ebf2..32686c79c41d 100644
>--- a/drivers/vhost/vhost.c
>+++ b/drivers/vhost/vhost.c
>@@ -2848,9 +2848,19 @@ 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) {
>+		/* Since we have updated avail_idx, the following
>+		 * call to vhost_get_vq_desc() will read available
>+		 * ring entries. Make sure that read happens after
>+		 * the avail_idx read.
>+		 */
>+		smp_rmb();
>+		return true;
>+	}
>
>-	return vq->avail_idx != vq->last_avail_idx;
>+	return false;
> }
> EXPORT_SYMBOL_GPL(vhost_enable_notify);
>
>-- 
>2.44.0
>