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Message-ID: <CACGkMEs-wUa_z_tGYEwBf7EVJAtuJdkX4HAdjqMXHEM1ys-gKQ@mail.gmail.com>
Date: Wed, 31 Jan 2024 17:12:22 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux.dev, Richard Weinberger <richard@....at>,
Anton Ivanov <anton.ivanov@...bridgegreys.com>, Johannes Berg <johannes@...solutions.net>,
"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>, Hans de Goede <hdegoede@...hat.com>,
Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
Vadim Pasternak <vadimp@...dia.com>, Bjorn Andersson <andersson@...nel.org>,
Mathieu Poirier <mathieu.poirier@...aro.org>, Cornelia Huck <cohuck@...hat.com>,
Halil Pasic <pasic@...ux.ibm.com>, Eric Farman <farman@...ux.ibm.com>,
Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>, Benjamin Berg <benjamin.berg@...el.com>,
Yang Li <yang.lee@...ux.alibaba.com>, linux-um@...ts.infradead.org,
netdev@...r.kernel.org, platform-driver-x86@...r.kernel.org,
linux-remoteproc@...r.kernel.org, linux-s390@...r.kernel.org,
kvm@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH vhost 04/17] virtio_ring: split: remove double check of
the unmap ops
On Tue, Jan 30, 2024 at 7:42 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> In the functions vring_unmap_one_split and
> vring_unmap_one_split_indirect,
> multiple checks are made whether unmap is performed and whether it is
> INDIRECT.
>
> These two functions are usually called in a loop, and we should put the
> check outside the loop.
>
> And we unmap the descs with VRING_DESC_F_INDIRECT on the same path with
> other descs, that make the thing more complex. If we distinguish the
> descs with VRING_DESC_F_INDIRECT before unmap, thing will be clearer.
>
> 1. only one desc of the desc table is used, we do not need the loop
> 2. the called unmap api is difference from the other desc
> 3. the vq->premapped is not needed to check
> 4. the vq->indirect is not needed to check
> 5. the state->indir_desc must not be null
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
> drivers/virtio/virtio_ring.c | 80 ++++++++++++++++++------------------
> 1 file changed, 39 insertions(+), 41 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index dd03bc5a81fe..2b41fdbce975 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -452,9 +452,6 @@ static void vring_unmap_one_split_indirect(const struct vring_virtqueue *vq,
> {
> u16 flags;
>
> - if (!vring_need_unmap_buffer(vq))
> - return;
> -
> flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
>
> dma_unmap_page(vring_dma_dev(vq),
> @@ -472,27 +469,12 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
>
> 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 (!vring_need_unmap_buffer(vq))
> - goto out;
> -
> - dma_unmap_page(vring_dma_dev(vq),
> - extra[i].addr,
> - extra[i].len,
> - (flags & VRING_DESC_F_WRITE) ?
> - DMA_FROM_DEVICE : DMA_TO_DEVICE);
> - }
> + dma_unmap_page(vring_dma_dev(vq),
> + extra[i].addr,
> + extra[i].len,
> + (flags & VRING_DESC_F_WRITE) ?
> + DMA_FROM_DEVICE : DMA_TO_DEVICE);
>
> -out:
> return extra[i].next;
> }
>
> @@ -660,7 +642,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> vq, desc, total_sg * sizeof(struct vring_desc),
> DMA_TO_DEVICE);
> if (vring_mapping_error(vq, addr)) {
> - if (vq->premapped)
> + if (!vring_need_unmap_buffer(vq))
> goto free_indirect;
>
> goto unmap_release;
> @@ -713,6 +695,9 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> return 0;
>
> unmap_release:
> +
> + WARN_ON(!vring_need_unmap_buffer(vq));
> +
> err_idx = i;
>
> if (indirect)
> @@ -774,34 +759,42 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> {
> unsigned int i, j;
> __virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> + u16 flags;
>
> /* Clear data ptr. */
> vq->split.desc_state[head].data = NULL;
> + flags = vq->split.desc_extra[head].flags;
>
> /* Put back on free list: unmap first-level descriptors and find end */
> i = head;
>
> - while (vq->split.vring.desc[i].flags & nextflag) {
> - vring_unmap_one_split(vq, i);
> - i = vq->split.desc_extra[i].next;
> - vq->vq.num_free++;
> - }
> -
> - vring_unmap_one_split(vq, i);
> - vq->split.desc_extra[i].next = vq->free_head;
> - vq->free_head = head;
> + if (!(flags & VRING_DESC_F_INDIRECT)) {
So during add we do:
if (!indirect && vring_need_unmap_buffer(vq))
vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
~VRING_DESC_F_NEXT;
Then using flags here unconditionally seems not reliable.
I post a patch to store flags unconditionally at:
https://lore.kernel.org/all/20220224122655-mutt-send-email-mst@kernel.org/
> + while (vq->split.vring.desc[i].flags & nextflag) {
> + if (vring_need_unmap_buffer(vq))
> + vring_unmap_one_split(vq, i);
> + i = vq->split.desc_extra[i].next;
> + vq->vq.num_free++;
> + }
>
> - /* Plus final descriptor */
> - vq->vq.num_free++;
> + if (vring_need_unmap_buffer(vq))
> + vring_unmap_one_split(vq, i);
>
> - if (vq->indirect) {
> + if (ctx)
> + *ctx = vq->split.desc_state[head].indir_desc;
> + } else {
> struct vring_desc *indir_desc =
> vq->split.desc_state[head].indir_desc;
> u32 len;
>
> - /* Free the indirect table, if any, now that it's unmapped. */
> - if (!indir_desc)
> - return;
> + if (vq->use_dma_api) {
> + struct vring_desc_extra *extra = vq->split.desc_extra;
> +
> + dma_unmap_single(vring_dma_dev(vq),
> + extra[i].addr,
> + extra[i].len,
> + (flags & VRING_DESC_F_WRITE) ?
> + DMA_FROM_DEVICE : DMA_TO_DEVICE);
> + }
Note that there's a following
BUG_ON(!(vq->split.desc_extra[head].flags &
VRING_DESC_F_INDIRECT));
Which I think we can remove.
Thanks
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