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

[Andy Lutomirski]

On Thu, Nov 22, 2018 at 8:56 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 22, 2018 at 2:32 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Random patch (with my "asm goto" hack included) attached, in case
> > > people want to play with it.
> >
> > Doesn't even look all that hacky to me. Any hack in it that I didn't
> > notice? :-)
>
> The code to use asm goto sadly doesn't have any fallback at all for
> the "no asm goto available".
>
> I guess we're getting close to "we require asm goto support", but I
> don't think we're there yet.

commit e501ce957a786ecd076ea0cfb10b114e6e4d0f40
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Wed Jan 17 11:42:07 2018 +0100

    x86: Force asm-goto

    We want to start using asm-goto to guarantee the absence of dynamic
    branches (and thus speculation).

    A primary prerequisite for this is of course that the compiler
    supports asm-goto. This effecively lifts the minimum GCC version to
    build an x86 kernel to gcc-4.5.

This is basically the only good outcome from the speculation crap as
far as I'm concerned :)

So I think your patch is viable.  Also, with that patch applied,
put_user_ex() should become worse than worthless -- if gcc is any
good, plain old:

if (unsafe_put_user(...) != 0)
  goto err;
if (unsafe_put_user(...) != 0)
  goto err;
etc.

will generate *better* code than a series of put_user_ex() calls.