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Tsirkin" To: Jason Wang Subject: Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed Message-ID: <20230808015921-mutt-send-email-mst@kernel.org> References: <20230808051110.3492693-1-yuanyaogoog@chromium.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: Tiwei Bie , Junichi Uekawa , LKML , virtualization@lists.linux-foundation.org, Yuan Yao , Takaya Saeki , Xuan Zhuo , "David S. 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2002:a17:906:c5:b0:994:555a:e49f with SMTP id 5-20020a17090600c500b00994555ae49fmr12096083eji.31.1691474388390; Mon, 07 Aug 2023 22:59:48 -0700 (PDT) Received: from redhat.com ([2.52.159.103]) by smtp.gmail.com with ESMTPSA id f25-20020a170906139900b0098d2d219649sm6214805ejc.174.2023.08.07.22.59.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 07 Aug 2023 22:59:47 -0700 (PDT) Date: Tue, 8 Aug 2023 01:59:43 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: Yuan Yao , LKML , Keiichi Watanabe , Daniel Verkamp , Takaya Saeki , Junichi Uekawa , "David S. Miller" , Tiwei Bie , Xuan Zhuo , virtualization@lists.linux-foundation.org Subject: Re: [PATCH] virtio_ring: fix avail_wrap_counter in virtqueue_add_packed Message-ID: <20230808015921-mutt-send-email-mst@kernel.org> References: <20230808051110.3492693-1-yuanyaogoog@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 08, 2023 at 01:43:02PM +0800, Jason Wang wrote: > On Tue, Aug 8, 2023 at 1:11 PM Yuan Yao wrote: > > > > In current packed virtqueue implementation, the avail_wrap_counter won't > > flip, in the case when the driver supplies a descriptor chain with a > > length equals to the queue size; total_sg == vq->packed.vring.num. > > > > Let’s assume the following situation: > > vq->packed.vring.num=4 > > vq->packed.next_avail_idx: 1 > > vq->packed.avail_wrap_counter: 0 > > > > Then the driver adds a descriptor chain containing 4 descriptors. > > > > We expect the following result with avail_wrap_counter flipped: > > vq->packed.next_avail_idx: 1 > > vq->packed.avail_wrap_counter: 1 > > > > But, the current implementation gives the following result: > > vq->packed.next_avail_idx: 1 > > vq->packed.avail_wrap_counter: 0 > > > > To reproduce the bug, you can set a packed queue size as small as > > possible, so that the driver is more likely to provide a descriptor > > chain with a length equal to the packed queue size. For example, in > > qemu run following commands: > > sudo qemu-system-x86_64 \ > > -enable-kvm \ > > -nographic \ > > -kernel "path/to/kernel_image" \ > > -m 1G \ > > -drive file="path/to/rootfs",if=none,id=disk \ > > -device virtio-blk,drive=disk \ > > -drive file="path/to/disk_image",if=none,id=rwdisk \ > > -device virtio-blk,drive=rwdisk,packed=on,queue-size=4,\ > > indirect_desc=off \ > > -append "console=ttyS0 root=/dev/vda rw init=/bin/bash" > > > > Inside the VM, create a directory and mount the rwdisk device on it. The > > rwdisk will hang and mount operation will not complete. > > > > This commit fixes the wrap counter error by flipping the > > packed.avail_wrap_counter, when start of descriptor chain equals to the > > end of descriptor chain (head == i). > > > > Fixes: 1ce9e6055fa0 ("virtio_ring: introduce packed ring support") > > Signed-off-by: Yuan Yao > > --- > > > > drivers/virtio/virtio_ring.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > index c5310eaf8b46..da1150d127c2 100644 > > --- a/drivers/virtio/virtio_ring.c > > +++ b/drivers/virtio/virtio_ring.c > > @@ -1461,7 +1461,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq, > > } > > } > > > > - if (i < head) > > + if (i <= head) > > vq->packed.avail_wrap_counter ^= 1; > > Would it be better to move the flipping to the place where we flip > avail_used_flags? I think I prefer this patch for stable, refactoring can be done on top. > if ((unlikely(++i >= vq->packed.vring.num))) { > i = 0; > vq->packed.avail_used_flags ^= > 1 << VRING_PACKED_DESC_F_AVAIL | > 1 << VRING_PACKED_DESC_F_USED; > } > > Thanks > > > > > /* We're using some buffers from the free list. */ > > -- > > 2.41.0.640.ga95def55d0-goog > >