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Message-ID: <1726126586.5406406-2-xuanzhuo@linux.alibaba.com>
Date: Thu, 12 Sep 2024 15:36:26 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: netdev@...r.kernel.org,
"Michael S. Tsirkin" <mst@...hat.com>,
Eugenio Pérez <eperezma@...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>,
virtualization@...ts.linux.dev,
bpf@...r.kernel.org
Subject: Re: [PATCH net-next 04/13] virtio_ring: perform premapped operations based on per-buffer
On Wed, 11 Sep 2024 11:54:25 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Tue, Aug 20, 2024 at 3:33 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > The current configuration sets the virtqueue (vq) to premapped mode,
> > implying that all buffers submitted to this queue must be mapped ahead
> > of time. This presents a challenge for the virtnet send queue (sq): the
> > virtnet driver would be required to keep track of dma information for vq
> > size * 17, which can be substantial. However, if the premapped mode were
> > applied on a per-buffer basis, the complexity would be greatly reduced.
> > With AF_XDP enabled, AF_XDP buffers would become premapped, while kernel
> > skb buffers could remain unmapped.
>
> Is this only applied to TX or both TX and RX.
For rx, if you mean per-buffer dma buffer, I think it is yes,
rx can reuse this. If you mean should we do premapped for the
normal rx buffers, I think we should, that can reduce the
dma map operations.
>
> >
> > We can distinguish them by sg_page(sg), When sg_page(sg) is NULL, this
> > indicates that the driver has performed DMA mapping in advance, allowing
> > the Virtio core to directly utilize sg_dma_address(sg) without
> > conducting any internal DMA mapping.
>
> This seems conflict with the code below?
>
> #define sg_is_premapped(sg) (!sg_page(sg))
Sorry, I do not get for you.
The key point is that the sg->page is setted by driver.
So I mean if the driver sets sg->page = NULL, then for this sg,
the virtio core can skip dma mapping. If the driver sets
sg->page to the page of the buffer, then the virtio core should
do dma mapping for this sg.
>
> And rethink of the design, a question is that is there a chance that
> VM_PFNMAP area could be used for virtio-net? If it is true, the trick
> for sg_page() can not work?
>
> A quick glance told me AF_XEP seems to be safe as it uses
> pin_user_pages(), but we need to check other possibilities.
>
> Or we need to fall back to our previous idea, having new APIs.
>
> > Additionally, DMA unmap operations
> > for this buffer will be bypassed.
> >
> > Suggested-by: Jason Wang <jasowang@...hat.com>
> > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > ---
> > drivers/virtio/virtio_ring.c | 70 +++++++++++++++++++++---------------
> > 1 file changed, 41 insertions(+), 29 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index b43eca93015c..7efddc71af67 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -235,6 +235,7 @@ static void vring_free(struct virtqueue *_vq);
> > */
> >
> > #define to_vvq(_vq) container_of_const(_vq, struct vring_virtqueue, vq)
> > +#define sg_is_premapped(sg) (!sg_page(sg))
> >
> > static bool virtqueue_use_indirect(const struct vring_virtqueue *vq,
> > unsigned int total_sg)
> > @@ -292,9 +293,10 @@ static bool vring_use_dma_api(const struct virtio_device *vdev)
> > return false;
> > }
> >
> > -static bool vring_need_unmap_buffer(const struct vring_virtqueue *vring)
> > +static bool vring_need_unmap_buffer(const struct vring_virtqueue *vring,
> > + const struct vring_desc_extra *extra)
> > {
> > - return vring->use_dma_api && !vring->premapped;
> > + return vring->use_dma_api && (extra->addr != DMA_MAPPING_ERROR);
> > }
> >
> > size_t virtio_max_dma_size(const struct virtio_device *vdev)
> > @@ -366,7 +368,7 @@ static struct device *vring_dma_dev(const struct vring_virtqueue *vq)
> > static int vring_map_one_sg(const struct vring_virtqueue *vq, struct scatterlist *sg,
> > enum dma_data_direction direction, dma_addr_t *addr)
> > {
> > - if (vq->premapped) {
> > + if (sg_is_premapped(sg)) {
> > *addr = sg_dma_address(sg);
> > return 0;
> > }
> > @@ -457,7 +459,7 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
> > (flags & VRING_DESC_F_WRITE) ?
> > DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > } else {
> > - if (!vring_need_unmap_buffer(vq))
> > + if (!vring_need_unmap_buffer(vq, extra))
> > goto out;
> >
> > dma_unmap_page(vring_dma_dev(vq),
> > @@ -510,7 +512,7 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
> > dma_addr_t addr,
> > unsigned int len,
> > u16 flags,
> > - bool indirect)
> > + bool indirect, bool premapped)
> > {
> > u16 next;
> >
> > @@ -518,7 +520,7 @@ static inline unsigned int virtqueue_add_desc_split(struct virtqueue *vq,
> > desc[i].addr = cpu_to_virtio64(vq->vdev, addr);
> > desc[i].len = cpu_to_virtio32(vq->vdev, len);
> >
> > - extra[i].addr = addr;
> > + extra[i].addr = premapped ? DMA_MAPPING_ERROR : addr;
> > extra[i].len = len;
> > extra[i].flags = flags;
> >
> > @@ -611,7 +613,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > */
> > i = virtqueue_add_desc_split(_vq, desc, extra, i, addr, sg->length,
> > VRING_DESC_F_NEXT,
> > - indirect);
> > + indirect, sg_is_premapped(sg));
> > }
> > }
> > for (; n < (out_sgs + in_sgs); n++) {
> > @@ -629,12 +631,12 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > sg->length,
> > VRING_DESC_F_NEXT |
> > VRING_DESC_F_WRITE,
> > - indirect);
> > + indirect, sg_is_premapped(sg));
> > }
> > }
> > /* Last one doesn't continue. */
> > desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> > - if (!indirect && vring_need_unmap_buffer(vq))
> > + if (!indirect && vring_need_unmap_buffer(vq, &extra[prev]))
> > vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
> > ~VRING_DESC_F_NEXT;
> >
> > @@ -643,19 +645,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > dma_addr_t addr = vring_map_single(
> > vq, desc, total_sg * sizeof(struct vring_desc),
> > DMA_TO_DEVICE);
> > - if (vring_mapping_error(vq, addr)) {
> > - if (vq->premapped)
> > - goto free_indirect;
> > -
> > + if (vring_mapping_error(vq, addr))
> > goto unmap_release;
> > - }
> >
> > virtqueue_add_desc_split(_vq, vq->split.vring.desc,
> > vq->split.desc_extra,
> > head, addr,
> > total_sg * sizeof(struct vring_desc),
> > VRING_DESC_F_INDIRECT,
> > - false);
> > + false, false);
> > }
> >
> > /* We're using some buffers from the free list. */
> > @@ -712,7 +710,6 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > i = vring_unmap_one_split(vq, &extra[i]);
> > }
> >
> > -free_indirect:
> > if (indirect)
> > kfree(desc);
> >
> > @@ -794,7 +791,7 @@ 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 (vring_need_unmap_buffer(vq)) {
> > + if (vq->use_dma_api) {
> > for (j = 0; j < len / sizeof(struct vring_desc); j++)
> > vring_unmap_one_split(vq, &extra[j]);
> > }
> > @@ -1228,7 +1225,7 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > (flags & VRING_DESC_F_WRITE) ?
> > DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > } else {
> > - if (!vring_need_unmap_buffer(vq))
> > + if (!vring_need_unmap_buffer(vq, extra))
> > return;
> >
> > dma_unmap_page(vring_dma_dev(vq),
> > @@ -1309,7 +1306,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > i++;
> >
> > if (unlikely(vq->use_dma_api)) {
> > - extra[i].addr = addr;
> > + extra[i].addr = sg_is_premapped(sg) ? DMA_MAPPING_ERROR : addr;
> > extra[i].len = sg->length;
> > extra[i].flags = n < out_sgs ? 0 : VRING_DESC_F_WRITE;
> > }
> > @@ -1320,12 +1317,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > addr = vring_map_single(vq, desc,
> > total_sg * sizeof(struct vring_packed_desc),
> > DMA_TO_DEVICE);
> > - if (vring_mapping_error(vq, addr)) {
> > - if (vq->premapped)
> > - goto free_desc;
> > -
> > + if (vring_mapping_error(vq, addr))
> > goto unmap_release;
> > - }
> >
> > vq->packed.vring.desc[head].addr = cpu_to_le64(addr);
> > vq->packed.vring.desc[head].len = cpu_to_le32(total_sg *
> > @@ -1383,7 +1376,6 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > for (i = 0; i < err_idx; i++)
> > vring_unmap_extra_packed(vq, &extra[i]);
> >
> > -free_desc:
> > kfree(desc);
> >
> > END_USE(vq);
> > @@ -1474,7 +1466,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > desc[i].id = cpu_to_le16(id);
> >
> > if (unlikely(vq->use_dma_api)) {
> > - vq->packed.desc_extra[curr].addr = addr;
> > + vq->packed.desc_extra[curr].addr = sg_is_premapped(sg) ?
> > + DMA_MAPPING_ERROR : addr;
> > vq->packed.desc_extra[curr].len = sg->length;
> > vq->packed.desc_extra[curr].flags =
> > le16_to_cpu(flags);
> > @@ -1625,10 +1618,9 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > if (!extra)
> > return;
> >
> > - if (vring_need_unmap_buffer(vq)) {
> > + if (vq->use_dma_api) {
> > len = vq->packed.desc_extra[id].len;
> > - for (i = 0; i < len / sizeof(struct vring_packed_desc);
> > - i++)
> > + for (i = 0; i < len / sizeof(struct vring_packed_desc); i++)
>
> Unnecessary changes?
Will fix.
Thanks.
>
>
> > vring_unmap_extra_packed(vq, &extra[i]);
> > }
> > kfree(desc);
> > @@ -2212,6 +2204,11 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> > * @data: the token identifying the buffer.
> > * @gfp: how to do memory allocations (if necessary).
> > *
> > + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> > + * mapping in advance, allowing the virtio core to directly utilize
> > + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> > + * DMA unmap operations for this buffer will be bypassed.
> > + *
> > * Caller must ensure we don't call this with other virtqueue operations
> > * at the same time (except where noted).
> > *
> > @@ -2246,6 +2243,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_sgs);
> > * @data: the token identifying the buffer.
> > * @gfp: how to do memory allocations (if necessary).
> > *
> > + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> > + * mapping in advance, allowing the virtio core to directly utilize
> > + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> > + * DMA unmap operations for this buffer will be bypassed.
> > + *
> > * Caller must ensure we don't call this with other virtqueue operations
> > * at the same time (except where noted).
> > *
> > @@ -2268,6 +2270,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_outbuf);
> > * @data: the token identifying the buffer.
> > * @gfp: how to do memory allocations (if necessary).
> > *
> > + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> > + * mapping in advance, allowing the virtio core to directly utilize
> > + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> > + * DMA unmap operations for this buffer will be bypassed.
> > + *
> > * Caller must ensure we don't call this with other virtqueue operations
> > * at the same time (except where noted).
> > *
> > @@ -2291,6 +2298,11 @@ EXPORT_SYMBOL_GPL(virtqueue_add_inbuf);
> > * @ctx: extra context for the token
> > * @gfp: how to do memory allocations (if necessary).
> > *
> > + * When sg_page(sg) is NULL, this indicates that the driver has performed DMA
> > + * mapping in advance, allowing the virtio core to directly utilize
> > + * sg_dma_address(sg) without conducting any internal DMA mapping. Additionally,
> > + * DMA unmap operations for this buffer will be bypassed.
> > + *
> > * Caller must ensure we don't call this with other virtqueue operations
> > * at the same time (except where noted).
> > *
> > --
> > 2.32.0.3.g01195cf9f
> >
>
> Thanks
>
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