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

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Date:   Wed, 21 Nov 2018 14:38:17 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Dave Hansen <dave.hansen@...el.com>,
        pabeni@...hat.com, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Brian Gerst <brgerst@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes

On Wed, Nov 21, 2018 at 10:44 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, Nov 21, 2018 at 10:26 AM Andy Lutomirski <luto@...capital.net> 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.