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

Xuan Zhuo
2024-11-06T05:58:44Z

On Wed, 6 Nov 2024 09:44:39 +0800, Jason Wang <jasowang@redhat.com> wrote:
> 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.

I see, we can do it in future.

Thanks.

>
> Thanks
>