netdev - Re: [PATCH vhost v4 03/10] virtio_ring: packed: structure the indirect desc table

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Message-ID: <1711093912.1488938-1-xuanzhuo@linux.alibaba.com>
Date: Fri, 22 Mar 2024 15:51:52 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.com>
Cc: virtualization@...ts.linux.dev,
 "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>,
 netdev@...r.kernel.org
Subject: Re: [PATCH vhost v4 03/10] virtio_ring: packed: structure the indirect desc table

On Fri, 22 Mar 2024 13:15:10 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Thu, Mar 21, 2024 at 4:29 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> >
> > On Thu, 21 Mar 2024 12:47:18 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > On Tue, Mar 12, 2024 at 11:36 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > > >
> > > > This commit structure the indirect desc table.
> > > > Then we can get the desc num directly when doing unmap.
> > > >
> > > > And save the dma info to the struct, then the indirect
> > > > will not use the dma fields of the desc_extra. The subsequent
> > > > commits will make the dma fields are optional. But for
> > > > the indirect case, we must record the dma info.
> > > >
> > > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > > ---
> > > >  drivers/virtio/virtio_ring.c | 66 +++++++++++++++++++++---------------
> > > >  1 file changed, 38 insertions(+), 28 deletions(-)
> > > >
> > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > index 0dfbd17e5a87..22a588bba166 100644
> > > > --- a/drivers/virtio/virtio_ring.c
> > > > +++ b/drivers/virtio/virtio_ring.c
> > > > @@ -72,9 +72,16 @@ struct vring_desc_state_split {
> > > >         struct vring_desc *indir_desc;  /* Indirect descriptor, if any. */
> > > >  };
> > > >
> > > > +struct vring_packed_desc_indir {
> > > > +       dma_addr_t addr;                /* Descriptor Array DMA addr. */
> > > > +       u32 len;                        /* Descriptor Array length. */
> > > > +       u32 num;
> > > > +       struct vring_packed_desc desc[];
> > > > +};
> > > > +
> > > >  struct vring_desc_state_packed {
> > > >         void *data;                     /* Data for callback. */
> > > > -       struct vring_packed_desc *indir_desc; /* Indirect descriptor, if any. */
> > > > +       struct vring_packed_desc_indir *indir_desc; /* Indirect descriptor, if any. */
> > >
> > > Maybe it's better just to have a vring_desc_extra here.
> >
> >
> > Do you mean replacing vring_packed_desc_indir by vring_desc_extra?
>
> Just add a vring_desc_extra in vring_desc_state_packed.
>
> >
> > I am ok for that. But vring_desc_extra has two extra items:
> >
> >         u16 flags;                      /* Descriptor flags. */
> >         u16 next;                       /* The next desc state in a list. */
> >
> > vring_packed_desc_indir has "desc". I think that is more convenient.
> >
> > So, I think vring_packed_desc_indir is appropriate.
>
> It reuses the existing structure so we had the chance to reuse the
> helper.

Do you mean vring_unmap_extra_packed()?

After last commit(virtio_ring: packed: remove double check of the unmap ops):

	/* caller must check vring_need_unmap_buffer() */
	static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
					     const struct vring_desc_extra *extra)
	{
		u16 flags;

		flags = extra->flags;

		dma_unmap_page(vring_dma_dev(vq),
			       extra->addr, extra->len,
			       (flags & VRING_DESC_F_WRITE) ?
			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
	}

But we should call dma_unmap_single() for indirect desc.

We know, dma_unmap_single() and dma_unmap_page() are same in essence.
So if we call dma_unmap_page for the indirect desc, we can reuse
this function. But I do not prefer doing this.

Thanks.


> And it could be used for future chained indirect (if it turns
> out to be necessary).
>
> Thanks
>
> > Or I missed something.
> >
> >
> > Thanks.
> >
> >
> > >
> > > Thanks
> > >
> >
>
>