linux-kernel - Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption

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Message-ID: <CAHk-=whsgA+8XtqJY91gqHhh9xLYQLM3kLLFTby=uf2eoZyK7Q@mail.gmail.com>
Date:   Sat, 6 Jul 2019 14:41:22 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        He Zhe <zhe.he@...driver.com>,
        Joel Fernandes <joel@...lfernandes.org>, devel@...ukata.com
Subject: Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption

On Fri, Jul 5, 2019 at 6:50 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Also; all previous attempts at fixing this have been about pushing the
> read_cr2() earlier; notably:
>
>   0ac09f9f8cd1 ("x86, trace: Fix CR2 corruption when tracing page faults")
>   d4078e232267 ("x86, trace: Further robustify CR2 handling vs tracing")

I think both of those are because people - again - felt like tracing
can validly corrupt CPU state, and then they fix up the symptoms
rather than the cause.

Which I disagree with.

> And I'm thinking that with exception of this patch, the rest are
> worthwhile cleanups regardless.

I don't have any issues with the patches themselves, I agree that they
are probably good on their own.

I *do* have issues with the "tracing can change CPU state so we need
to deal with it" model, though. I think that mode of thinking is
wrong. I don't believe tracing should ever change core CPU state and
that be considered ok.

> Also; while looking at this, if we do continue with the C wrappers from
> the very last patch, we can do horrible things like this on top and move
> the read_cr2() back into C code.

Again, I don't think that is the problem. I think it's a much more
fundamental problem in thinking that core code should be changed
because tracing is broken garbage and didn't do things right.

I see Eiichi's patch, and it makes me go "that looks better" - simply
because it fixes the fundamental issue, rather than working around the
symptoms.

               Linus