Subject: Re: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 13 Dec 2018 10:27:09 -0500

On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote:
> Hi:
> 
> This series tries to access virtqueue metadata through kernel virtual
> address instead of copy_user() friends since they had too much
> overheads like checks, spec barriers or even hardware feature
> toggling.

Userspace accesses through remapping tricks and next time there's a need
for a new barrier we are left to figure it out by ourselves.  I don't
like the idea I have to say.  As a first step, why don't we switch to
unsafe_put_user/unsafe_get_user etc?
That would be more of an apples to apples comparison, would it not?

> Test shows about 24% improvement on TX PPS. It should benefit other
> cases as well.
> 
> Please review
> 
> Jason Wang (3):
>   vhost: generalize adding used elem
>   vhost: fine grain userspace memory accessors
>   vhost: access vq metadata through kernel virtual address
> 
>  drivers/vhost/vhost.c | 281 ++++++++++++++++++++++++++++++++++++++----
>  drivers/vhost/vhost.h |  11 ++
>  2 files changed, 266 insertions(+), 26 deletions(-)
> 
> -- 
> 2.17.1