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Message-ID: <1706695107.9558825-2-xuanzhuo@linux.alibaba.com>
Date: Wed, 31 Jan 2024 17:58:27 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.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 02/17] virtio_ring: packed: remove double check of the unmap ops
On Wed, 31 Jan 2024 17:12:19 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Tue, Jan 30, 2024 at 7:42 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > In the functions vring_unmap_extra_packed and vring_unmap_desc_packed,
> > 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 | 76 ++++++++++++++++++------------------
> > 1 file changed, 39 insertions(+), 37 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 4677831e6c26..7280a1706cca 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -1220,6 +1220,7 @@ static u16 packed_last_used(u16 last_used_idx)
> > return last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
> > }
> >
> > +/* caller must check vring_need_unmap_buffer() */
> > static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> > const struct vring_desc_extra *extra)
> > {
> > @@ -1227,33 +1228,18 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> >
> > 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 (!vring_need_unmap_buffer(vq))
> > - return;
> > -
> > - dma_unmap_page(vring_dma_dev(vq),
> > - extra->addr, extra->len,
> > - (flags & VRING_DESC_F_WRITE) ?
> > - DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > - }
> > + dma_unmap_page(vring_dma_dev(vq),
> > + extra->addr, extra->len,
> > + (flags & VRING_DESC_F_WRITE) ?
> > + DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > }
> >
> > +/* caller must check vring_need_unmap_buffer() */
> > static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> > const struct vring_packed_desc *desc)
> > {
> > u16 flags;
> >
> > - if (!vring_need_unmap_buffer(vq))
> > - return;
> > -
> > flags = le16_to_cpu(desc->flags);
> >
> > dma_unmap_page(vring_dma_dev(vq),
> > @@ -1329,7 +1315,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > total_sg * sizeof(struct vring_packed_desc),
> > DMA_TO_DEVICE);
> > if (vring_mapping_error(vq, addr)) {
> > - if (vq->premapped)
> > + if (!vring_need_unmap_buffer(vq))
> > goto free_desc;
> >
> > goto unmap_release;
> > @@ -1344,10 +1330,11 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > vq->packed.desc_extra[id].addr = addr;
> > vq->packed.desc_extra[id].len = total_sg *
> > sizeof(struct vring_packed_desc);
> > - vq->packed.desc_extra[id].flags = VRING_DESC_F_INDIRECT |
> > - vq->packed.avail_used_flags;
> > }
> >
> > + vq->packed.desc_extra[id].flags = VRING_DESC_F_INDIRECT |
> > + vq->packed.avail_used_flags;
>
> Is this a bug fix? Or if we only need to check _F_INDIRECT, we can
> simply avoid doing this by checking vq->indirect && state->indir_desc?
>
> > +
> > /*
> > * A driver MUST NOT make the first descriptor in the list
> > * available before all subsequent descriptors comprising
> > @@ -1388,6 +1375,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> > unmap_release:
> > err_idx = i;
> >
> > + WARN_ON(!vring_need_unmap_buffer(vq));
>
> Nitpick, using BUG_ON might be better as it may lead to unexpected
> results which we can't recover from.
the checkpatch.pl does not like BUG_ON.
I have not preference.
>
> > +
> > for (i = 0; i < err_idx; i++)
> > vring_unmap_desc_packed(vq, &desc[i]);
> >
> > @@ -1484,9 +1473,10 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > if (unlikely(vring_need_unmap_buffer(vq))) {
> > vq->packed.desc_extra[curr].addr = addr;
> > vq->packed.desc_extra[curr].len = sg->length;
> > - vq->packed.desc_extra[curr].flags =
> > - le16_to_cpu(flags);
> > }
> > +
> > + vq->packed.desc_extra[curr].flags = le16_to_cpu(flags);
> > +
> > prev = curr;
> > curr = vq->packed.desc_extra[curr].next;
> >
> > @@ -1536,6 +1526,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >
> > vq->packed.avail_used_flags = avail_used_flags;
> >
> > + WARN_ON(!vring_need_unmap_buffer(vq));
> > +
> > for (n = 0; n < total_sg; n++) {
> > if (i == err_idx)
> > break;
> > @@ -1605,7 +1597,9 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > struct vring_desc_state_packed *state = NULL;
> > struct vring_packed_desc *desc;
> > unsigned int i, curr;
> > + u16 flags;
> >
> > + flags = vq->packed.desc_extra[id].flags;
> > state = &vq->packed.desc_state[id];
> >
> > /* Clear data ptr. */
> > @@ -1615,22 +1609,32 @@ static void detach_buf_packed(struct vring_virtqueue *vq,
> > vq->free_head = id;
> > vq->vq.num_free += state->num;
> >
> > - if (unlikely(vring_need_unmap_buffer(vq))) {
> > - curr = id;
> > - for (i = 0; i < state->num; i++) {
> > - vring_unmap_extra_packed(vq,
> > - &vq->packed.desc_extra[curr]);
> > - curr = vq->packed.desc_extra[curr].next;
> > + if (!(flags & VRING_DESC_F_INDIRECT)) {
> > + if (vring_need_unmap_buffer(vq)) {
> > + curr = id;
> > + for (i = 0; i < state->num; i++) {
> > + vring_unmap_extra_packed(vq,
> > + &vq->packed.desc_extra[curr]);
> > + curr = vq->packed.desc_extra[curr].next;
> > + }
>
> So before the change, we had:
>
> if (unlikely(vq->do_unmap)) {
> curr = id;
> for (i = 0; i < state->num; i++) {
> vring_unmap_extra_packed(vq,
> &vq->packed.desc_extra[curr]);
> curr = vq->packed.desc_extra[curr].next;
> }
> }
>
> This looks like a bug as we should unmap the indirect descriptor
> regradless of whether do_unmap is true or false.
>
> If yes, we need a independent fix instead of squashing it in this patch?
YES. I noticed this.
I will post a fix to the stable branch.
Thanks.
>
> > }
> > - }
> >
> > - if (vq->indirect) {
> > + if (ctx)
> > + *ctx = state->indir_desc;
> > + } else {
> > + const struct vring_desc_extra *extra;
> > u32 len;
> >
> > + if (vq->use_dma_api) {
> > + extra = &vq->packed.desc_extra[id];
> > + dma_unmap_single(vring_dma_dev(vq),
> > + extra->addr, extra->len,
> > + (flags & VRING_DESC_F_WRITE) ?
> > + DMA_FROM_DEVICE : DMA_TO_DEVICE);
> > + }
> > +
>
> Thanks
>
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