Re: [PATCH] x86: only use ERMS for user copies for larger sizes

[Andy Lutomirski]

On Wed, Nov 21, 2018 at 10:44 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 21, 2018 at 10:26 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> > Can we maybe use this as an excuse to ask for some reasonable instructions to access user memory?
>
> I did that long ago. It's why we have CLAC/STAC today. I was told that
> what I actually asked for (get an instruction to access user space - I
> suggested using a segment override prefix) was not viable.
>

Maybe it wasn't viable *then*, but WRUSS really does write to
userspace with user privilege.  Surely the same mechanism with some of
the weirdness removed would do what we want.