Re: [PATCH net-next v2 02/13] virtio_ring: split: record extras for indirect buffers

Jason Wang
2024-11-06T01:44:54Z

On Tue, Nov 5, 2024 at 2:53 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Tue, 5 Nov 2024 11:42:09 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Wed, Oct 30, 2024 at 4:25 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > >
> > > The subsequent commit needs to know whether every indirect buffer is
> > > premapped or not. So we need to introduce an extra struct for every
> > > indirect buffer to record this info.
> > >
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > ---
> > > drivers/virtio/virtio_ring.c | 112 ++++++++++++++++-------------------
> > > 1 file changed, 52 insertions(+), 60 deletions(-)
> >
> > Do we have a performance impact for this patch?
> >
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 97590c201aa2..dca093744fe1 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -69,7 +69,11 @@
> > >
> > > struct vring_desc_state_split {
> > > void *data; /* Data for callback. */
> > > - struct vring_desc *indir_desc; /* Indirect descriptor, if any. */
> > > +
> > > + /* Indirect extra table and desc table, if any. These two will be
> > > + * allocated together. So we won't stress more to the memory allocator.
> > > + */
> > > + struct vring_desc *indir_desc;
> >
> > So it looks like we put a descriptor table after the extra table. Can
> > this lead to more crossing page mappings for the indirect descriptors?
> >
> > If yes, it seems expensive so we probably need to make the descriptor
> > table come first.
>
> No, the descriptors are before extra table.

Well, you need then tweak the above comment, it said

"Indirect extra table and desc table".

> So, there is not performance impact.
>
>
> >
> > > };
> > >

[...]

> > > while (vq->split.vring.desc[i].flags & nextflag) {
> > > - vring_unmap_one_split(vq, i);
> > > + vring_unmap_one_split(vq, &extra[i]);
> >
> > Not sure if I've asked this before. But this part seems to deserve an
> > independent fix for -stable.
>
> What fix?

I meant for hardening we need to check the flags stored in the extra
instead of the descriptor itself as it could be mangled by the device.

Thanks