Re: [PULL] vhost: cleanups and fixes

[Linus Torvalds]

On Fri, Nov 2, 2018 at 6:04 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> I've tried making access_ok mask the parameter it gets.

PLEASE don't do this.

Just use "copy_to/from_user()".

We have had lots of bugs because code bitrots.

And no, the access_ok() checks aren't expensive, not even in a loop.
They *used* to be somewhat expensive compared to the access, but that
simply isn't true any more. The real expense in copy_to_user and
friends are in the user access bit setting (STAC and CLAC on x86),
which easily an order of magnitude more expensive than access_ok().

So just get rid of the double-underscore version. It's basically
always a mis-optimization due to entirely historical reasons. I can
pretty much guarantee that it's not visible in profiles.

                  Linus