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Date: Thu, 13 Jul 2023 13:45:23 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: virtualization@...ts.linux-foundation.org,
"Michael S. Tsirkin" <mst@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
netdev@...r.kernel.org,
bpf@...r.kernel.org,
Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v11 06/10] virtio_ring: skip unmap for premapped
On Thu, 13 Jul 2023 12:21:26 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Thu, Jul 13, 2023 at 12:06 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > On Thu, 13 Jul 2023 11:50:57 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > On Mon, Jul 10, 2023 at 11:42 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > >
> > > > Now we add a case where we skip dma unmap, the vq->premapped is true.
> > > >
> > > > We can't just rely on use_dma_api to determine whether to skip the dma
> > > > operation. For convenience, I introduced the "do_unmap". By default, it
> > > > is the same as use_dma_api. If the driver is configured with premapped,
> > > > then do_unmap is false.
> > > >
> > > > So as long as do_unmap is false, for addr of desc, we should skip dma
> > > > unmap operation.
> > > >
> > > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > > ---
> > > > drivers/virtio/virtio_ring.c | 42 ++++++++++++++++++++++++------------
> > > > 1 file changed, 28 insertions(+), 14 deletions(-)
> > > >
> > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > index 1fb2c6dca9ea..10ee3b7ce571 100644
> > > > --- a/drivers/virtio/virtio_ring.c
> > > > +++ b/drivers/virtio/virtio_ring.c
> > > > @@ -175,6 +175,11 @@ struct vring_virtqueue {
> > > > /* Do DMA mapping by driver */
> > > > bool premapped;
> > > >
> > > > + /* Do unmap or not for desc. Just when premapped is False and
> > > > + * use_dma_api is true, this is true.
> > > > + */
> > > > + bool do_unmap;
> > > > +
> > > > /* Head of free buffer list. */
> > > > unsigned int free_head;
> > > > /* Number we've added since last sync. */
> > > > @@ -440,7 +445,7 @@ static void vring_unmap_one_split_indirect(const struct vring_virtqueue *vq,
> > > > {
> > > > u16 flags;
> > > >
> > > > - if (!vq->use_dma_api)
> > > > + if (!vq->do_unmap)
> > > > return;
> > > >
> > > > flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> > > > @@ -458,18 +463,21 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
> > > > struct vring_desc_extra *extra = vq->split.desc_extra;
> > > > u16 flags;
> > > >
> > > > - if (!vq->use_dma_api)
> > > > - goto out;
> > > > -
> > > > flags = extra[i].flags;
> > > >
> > > > if (flags & VRING_DESC_F_INDIRECT) {
> > > > + if (!vq->use_dma_api)
> > > > + goto out;
> > > > +
> > > > dma_unmap_single(vring_dma_dev(vq),
> > > > extra[i].addr,
> > > > extra[i].len,
> > > > (flags & VRING_DESC_F_WRITE) ?
> > > > DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > > > } else {
> > > > + if (!vq->do_unmap)
> > > > + goto out;
> > > > +
> > > > dma_unmap_page(vring_dma_dev(vq),
> > > > extra[i].addr,
> > > > extra[i].len,
> > > > @@ -635,7 +643,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > > > }
> > > > /* Last one doesn't continue. */
> > > > desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> > > > - if (!indirect && vq->use_dma_api)
> > > > + if (!indirect && vq->do_unmap)
> > > > vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
> > > > ~VRING_DESC_F_NEXT;
> > > >
> > > > @@ -794,7 +802,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> > > > VRING_DESC_F_INDIRECT));
> > > > BUG_ON(len == 0 || len % sizeof(struct vring_desc));
> > > >
> > > > - if (vq->use_dma_api) {
> > > > + if (vq->do_unmap) {
> > > > for (j = 0; j < len / sizeof(struct vring_desc); j++)
> > > > vring_unmap_one_split_indirect(vq, &indir_desc[j]);
> > > > }
> > > > @@ -1217,17 +1225,20 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > > > {
> > > > u16 flags;
> > > >
> > > > - if (!vq->use_dma_api)
> > > > - return;
> > > > -
> > > > flags = extra->flags;
> > > >
> > > > if (flags & VRING_DESC_F_INDIRECT) {
> > > > + if (!vq->use_dma_api)
> > > > + return;
> > > > +
> > > > dma_unmap_single(vring_dma_dev(vq),
> > > > extra->addr, extra->len,
> > > > (flags & VRING_DESC_F_WRITE) ?
> > > > DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > > > } else {
> > > > + if (!vq->do_unmap)
> > > > + return;
> > >
> > > This seems not straightforward than:
> > >
> > > if (!vq->use_dma_api)
> > > return;
> > >
> > > if (INDIRECT) {
> > > } else if (!vq->premapped) {
> > > }
> > >
> > > ?
> >
> >
> > My logic here is that for the real buffer, we use do_unmap to judge uniformly.
> > And indirect still use use_dma_api to judge.
> >
> > From this point of view, how do you feel?
>
> We can hear from others but a state machine with three booleans seems
> not easy for me to read.
Yes, I also think too many booleans, so I introduce do_unmap, then
for the real buffer(not the indirect desc array), we just check do_unmap.
Thanks.
>
> Thanks
>
> >
> > Thanks.
> >
> >
> > >
> > > Thanks
> > >
> > > > +
> > > > dma_unmap_page(vring_dma_dev(vq),
> > > > extra->addr, extra->len,
> > > > (flags & VRING_DESC_F_WRITE) ?
> > > > @@ -1240,7 +1251,7 @@ static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> > > > {
> > > > u16 flags;
> > > >
> > > > - if (!vq->use_dma_api)
> > > > + if (!vq->do_unmap)
> > > > return;
> > > >
> > > > flags = le16_to_cpu(desc->flags);
> > > > @@ -1329,7 +1340,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > > > sizeof(struct vring_packed_desc));
> > > > vq->packed.vring.desc[head].id = cpu_to_le16(id);
> > > >
> > > > - if (vq->use_dma_api) {
> > > > + if (vq->do_unmap) {
> > > > vq->packed.desc_extra[id].addr = addr;
> > > > vq->packed.desc_extra[id].len = total_sg *
> > > > sizeof(struct vring_packed_desc);
> > > > @@ -1470,7 +1481,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > > desc[i].len = cpu_to_le32(sg->length);
> > > > desc[i].id = cpu_to_le16(id);
> > > >
> > > > - if (unlikely(vq->use_dma_api)) {
> > > > + if (unlikely(vq->do_unmap)) {
> > > > vq->packed.desc_extra[curr].addr = addr;
> > > > vq->packed.desc_extra[curr].len = sg->length;
> > > > vq->packed.desc_extra[curr].flags =
> > > > @@ -1604,7 +1615,7 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > > > vq->free_head = id;
> > > > vq->vq.num_free += state->num;
> > > >
> > > > - if (unlikely(vq->use_dma_api)) {
> > > > + if (unlikely(vq->do_unmap)) {
> > > > curr = id;
> > > > for (i = 0; i < state->num; i++) {
> > > > vring_unmap_extra_packed(vq,
> > > > @@ -1621,7 +1632,7 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > > > if (!desc)
> > > > return;
> > > >
> > > > - if (vq->use_dma_api) {
> > > > + if (vq->do_unmap) {
> > > > len = vq->packed.desc_extra[id].len;
> > > > for (i = 0; i < len / sizeof(struct vring_packed_desc);
> > > > i++)
> > > > @@ -2080,6 +2091,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
> > > > vq->dma_dev = dma_dev;
> > > > vq->use_dma_api = vring_use_dma_api(vdev);
> > > > vq->premapped = false;
> > > > + vq->do_unmap = vq->use_dma_api;
> > > >
> > > > vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> > > > !context;
> > > > @@ -2587,6 +2599,7 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > > > vq->dma_dev = dma_dev;
> > > > vq->use_dma_api = vring_use_dma_api(vdev);
> > > > vq->premapped = false;
> > > > + vq->do_unmap = vq->use_dma_api;
> > > >
> > > > vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> > > > !context;
> > > > @@ -2765,6 +2778,7 @@ int virtqueue_set_premapped(struct virtqueue *_vq)
> > > > return -EINVAL;
> > > >
> > > > vq->premapped = true;
> > > > + vq->do_unmap = false;
> > > >
> > > > return 0;
> > > > }
> > > > --
> > > > 2.32.0.3.g01195cf9f
> > > >
> > >
> >
>
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