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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 V3 14/19] virtio_ring: determine descriptor flags at one time Message-ID: <20250701023157-mutt-send-email-mst@kernel.org> References: <20250616082518.10411-1-jasowang@redhat.com> <20250616082518.10411-15-jasowang@redhat.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: <20250616082518.10411-15-jasowang@redhat.com> On Mon, Jun 16, 2025 at 04:25:12PM +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. > > Reviewed-by: Xuan Zhuo > Signed-off-by: Jason Wang > --- > drivers/virtio/virtio_ring.c | 25 +++++++++++++------------ > 1 file changed, 13 insertions(+), 12 deletions(-) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index af32d1a1a1db..d5e4d4cd2487 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -570,7 +570,7 @@ 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, c, avail, descs_used, err_idx; > int head; > bool indirect; > > @@ -626,46 +626,47 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq, > return -ENOSPC; > } > > + c = 0; initialize at point of declaration? > 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; > + u16 flags = 0; > > if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped)) > goto unmap_release; > > - prev = i; > + if (++c != total_sg) > + flags = VRING_DESC_F_NEXT; > + Don't like it how the logic is split. flags isn't used before that. So I prefer: flags = ++c == total_sg ? 0 : VRING_DESC_F_NEXT; and at that point, we do not really need flags anymore: > /* 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, > + flags, So just: i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len, ++c == total_sg ? 0 : VRING_DESC_F_NEXT, here and we are done. > premapped); > } > } > for (; n < (out_sgs + in_sgs); n++) { > for (sg = sgs[n]; sg; sg = sg_next(sg)) { > + u16 flags = VRING_DESC_F_WRITE; > dma_addr_t addr; > u32 len; > > if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped)) > goto unmap_release; > > - prev = i; > + if (++c != total_sg) > + flags |= VRING_DESC_F_NEXT; > + Don't like it that above it's "=" here it is "|=". And flags isn't used before that. So I prefer: flags = ++c == total_sg ? VRING_DESC_F_WRITE : VRING_DESC_F_WRITE | VRING_DESC_F_NEXT; and again we don't really need the variable: > /* 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, so just: i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len, (++c == total_sg ? 0 : VRING_DESC_F_NEXT) | VRING_DESC_F_WRITE, is clearer, and the patch smaller. > - premapped); > + flags, 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. */ > -- > 2.34.1