linux-kernel - Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

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Date:   Thu, 30 Mar 2017 18:33:55 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Stas Sergeev <stsp@...t.ru>,
        Alexandre Julliard <julliard@...ehq.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        Alexandre Julliard <julliard@...ehq.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, linux-msdos@...r.kernel.org,
        wine-devel@...ehq.org
Subject: Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

On Thu, 2017-03-30 at 13:10 +0300, Stas Sergeev wrote:
> 30.03.2017 08:14, Ricardo Neri пишет:
> >>>> But at least dosemu implements it, so probably it is needed.
> >>> Right.
> >>>
> >>>> Of course if it is used by one of 100 DOS progs, then there
> >>>> is an option to just add its support to dosemu2 and pretend
> >>>> the compatibility problems did not exist. :)
> >>> Do you mean relaying the GP fault to dosemu instead of trapping it and
> >>> emulating it in the kernel?
> >> Yes, that would be optimal if this does not severely break
> >> the current setups. If we can find out that smsw is not in
> >> the real use, we can probably do exactly that.
> >> But other
> >> instructions are not in real use in v86 for sure, so I
> >> wouldn't be adding the explicit test-cases to the kernel
> >> that will make you depend on some particular behaviour
> >> that no one may need.
> >> My objection was that we shouldn't
> >> write tests before we know exactly how we want this to work.
> > OK, if only SMSW is used then I'll keep the emulation for SMSW only.
> In fact, smsw has an interesting property, which is that
> no one will ever want to disable its in-kernel emulation
> to provide its own.
> So while I'll try to estimate its usage, emulating it in kernel
> will not be that problematic in either case.

Ah good to know!

> As for protected mode, if wine only needs sgdt/sidt, then
> again, no one will want to disable its emulation. Not the
> case with sldt, but AFAICS wine doesn't need sldt, and so
> we can leave sldt without a fixups. Is my understanding
> correct?

This is my understanding as well. I could not find any use of sldt in
wine. Alexandre, would you mind confirming?

> In this case, I suppose, we are very well on a way to avoid
> the extra syscalls to toggle the emulation features.

Great! Then I will keep the emulation for sgdt, sidt, and smsw but not
for str and sldt; for both vm86 and protected mode. This seems to be the
agreement.

Thanks and BR,
Ricardo