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Tsirkin" To: Jason Wang Cc: xuanzhuo@linux.alibaba.com, eperezma@redhat.com, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH V6 14/19] virtio_ring: determine descriptor flags at one time Message-ID: <20250921134323-mutt-send-email-mst@kernel.org> References: <20250919073154.49278-1-jasowang@redhat.com> <20250919073154.49278-15-jasowang@redhat.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20250919073154.49278-15-jasowang@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Rqbm00EEzuqSsNwBpLZ8N12uVcAmsf4qWdPwluqsF_c_1758476646 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit On Fri, Sep 19, 2025 at 03:31:49PM +0800, Jason Wang wrote: > Let's determine the last descriptor by counting the number of sg. This > would be consistent with packed virtqueue implementation and ease the > future in-order implementation. > > Acked-by: Eugenio Pérez > Reviewed-by: Xuan Zhuo > Signed-off-by: Jason Wang > --- > drivers/virtio/virtio_ring.c | 21 ++++++++------------- > 1 file changed, 8 insertions(+), 13 deletions(-) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index 1045c553ee65..0949675a3d12 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -574,7 +574,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq, > struct vring_desc_extra *extra; > struct scatterlist *sg; > struct vring_desc *desc; > - unsigned int i, n, avail, descs_used, prev, err_idx; > + unsigned int i, n, avail, descs_used, err_idx, c = 0; > int head; > bool indirect; > > @@ -631,6 +631,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq, > } > > for (n = 0; n < out_sgs; n++) { > + sg = sgs[n]; > for (sg = sgs[n]; sg; sg = sg_next(sg)) { > dma_addr_t addr; > u32 len; How does this make any sense? > @@ -638,12 +639,12 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq, > if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped)) > goto unmap_release; > > - prev = i; > /* Note that we trust indirect descriptor > * table since it use stream DMA mapping. > */ > i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len, > - VRING_DESC_F_NEXT, > + ++c == total_sg ? > + 0 : VRING_DESC_F_NEXT, > premapped); > } > } > @@ -655,21 +656,15 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq, > if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped)) > goto unmap_release; > > - prev = i; > /* Note that we trust indirect descriptor > * table since it use stream DMA mapping. > */ > - i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len, > - VRING_DESC_F_NEXT | > - VRING_DESC_F_WRITE, > - premapped); > + i = virtqueue_add_desc_split(vq, desc, extra, > + i, addr, len, > + (++c == total_sg ? 0 : VRING_DESC_F_NEXT) | > + VRING_DESC_F_WRITE, premapped); > } > } > - /* Last one doesn't continue. */ > - desc[prev].flags &= cpu_to_virtio16(vq->vq.vdev, ~VRING_DESC_F_NEXT); > - if (!indirect && vring_need_unmap_buffer(vq, &extra[prev])) > - vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &= > - ~VRING_DESC_F_NEXT; > > if (indirect) { > /* Now that the indirect table is filled in, map it. */ > -- > 2.31.1