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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 21 Nov 2018 14:38:17 -0800
Message-ID: <CALCETrUWbZGg5=r_GeJzfrmZpuC9y3f4jxFoZuOm4aGuHGhDog@mail.gmail.com>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Dave Hansen <dave.hansen@intel.com>,
        pabeni@redhat.com, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrew Lutomirski <luto@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Brian Gerst <brgerst@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Nov 21, 2018 at 10:44 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, Nov 21, 2018 at 10:26 AM Andy Lutomirski <luto@amacapital.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.