Re: [PATCH net-next RFC 2/5] vhost: introduce helper to prefetch desc index

[Jason Wang]

On 2017å09æ28æ 06:57, Michael S. Tsirkin wrote:
> On Wed, Sep 27, 2017 at 08:35:47AM +0800, Jason Wang wrote:
> > On 2017å09æ27æ 03:19, Michael S. Tsirkin wrote:
> > > On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang wrote:
> > > > This patch introduces vhost_prefetch_desc_indices() which could batch
> > > > descriptor indices fetching and used ring updating. This intends to
> > > > reduce the cache misses of indices fetching and updating and reduce
> > > > cache line bounce when virtqueue is almost full. copy_to_user() was
> > > > used in order to benefit from modern cpus that support fast string
> > > > copy. Batched virtqueue processing will be the first user.
> > > >
> > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > > ---
> > > >    drivers/vhost/vhost.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >    drivers/vhost/vhost.h |  3 +++
> > > >    2 files changed, 58 insertions(+)
> > > >
> > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > index f87ec75..8424166d 100644
> > > > --- a/drivers/vhost/vhost.c
> > > > +++ b/drivers/vhost/vhost.c
> > > > @@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct vhost_dev *dev,
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
> > > > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> > > > +				struct vring_used_elem *heads,
> > > > +				u16 num, bool used_update)
> > > why do you need to combine used update with prefetch?
> > For better performance
>
> Why is sticking a branch in there better than requesting the update
> conditionally from the caller?

Ok, I get your point, I can split the two functions.

> > and I believe we don't care about the overhead when
> > we meet errors in tx.
> That's a separate question, I do not really understand how
> you can fetch a descriptor and update the used ring at the same
> time. This allows the guest to overwrite the buffer.
> I might be misunderstanding what is going on here though.

We don't update used idx, so guest can't overwrite the buffer I think?

Thanks