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Tue, 06 Oct 2026 03:10:30 -0700 (PDT) Date: Tue, 6 Oct 2026 06:10:27 -0400 From: "Michael S. Tsirkin" To: sungbyeongchan Cc: Jason Wang , Eugenio =?iso-8859-1?Q?P=E9rez?= , Xuan Zhuo , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH] virtio_ring: use shadow flags when detaching split descriptors Message-ID: <20261006060900-mutt-send-email-mst@kernel.org> References: <20261006100400.842345-1-tjdqudcks0424@naver.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261006100400.842345-1-tjdqudcks0424@naver.com> On Tue, Oct 06, 2026 at 07:04:00PM +0900, sungbyeongchan wrote: > Split virtqueues save descriptor flags and next indexes in desc_extra > before publishing descriptors to the device. The detach path uses the > shadow next index, but decides whether to continue by rereading the NEXT > flag from the shared descriptor. > > A device can change the flag after publication and make > detach_buf_split_in_order() detach an adjacent active descriptor. This > overcounts num_free and may put a descriptor on the free list twice. This seems neither here nor there I'd drop this paragraph. > Use the shadow flags for the continuation decision as well. This keeps > all detach metadata in the same driver-owned snapshot and avoids an > unnecessary shared-ring read. > > A VDUSE/virtio-vdpa test which changed NEXT after publication made six > valid completions return seven descriptors, with one descriptor ID > duplicated on refill. With this change, the same test returned six > unique descriptors, while the unmodified control remained unchanged. This too. > Fixes: 72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra for split virtqueue") > Assisted-by: LLM > Signed-off-by: sungbyeongchan > --- > drivers/virtio/virtio_ring.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) Patch itself is good, I always like it when the # of lines of code drops) Acked-by: Michael S. Tsirkin > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index db678f5a80e03..b4cb433df1dec 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -905,7 +905,6 @@ static unsigned detach_buf_split_in_order(struct vring_virtqueue *vq, > { > struct vring_desc_extra *extra; > unsigned int i; > - __virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT); > > /* Clear data ptr. */ > vq->split.desc_state[head].data = NULL; > @@ -915,7 +914,7 @@ static unsigned detach_buf_split_in_order(struct vring_virtqueue *vq, > /* Put back on free list: unmap first-level descriptors and find end */ > i = head; > > - while (vq->split.vring.desc[i].flags & nextflag) { > + while (extra[i].flags & VRING_DESC_F_NEXT) { > i = vring_unmap_one_split(vq, &extra[i]); > vq->vq.num_free++; > } > -- > 2.43.0