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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 5 Jul 2019 12:27:49 +0900
Message-ID: <CAHk-=whzQDR+hjhontUhsCbQOryU7VZ9K79EVjL2KCduZWDc1Q@mail.gmail.com>
Subject: Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Andrew Lutomirski <luto@kernel.org>,
        Peter Anvin <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Juergen Gross <jgross@suse.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        He Zhe <zhe.he@windriver.com>,
        Joel Fernandes <joel@joelfernandes.org>, devel@etsukata.com
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On Fri, Jul 5, 2019 at 12:16 PM Andy Lutomirski <luto@amacapital.net> wrote=
:
>
> If nothing else, MOV to CR2 is architecturally serializing, so, unless th=
ere=E2=80=99s some fancy unwinding involved, this will be quite slow.

That's why the NMI code does this:

        if (unlikely(this_cpu_read(nmi_cr2) !=3D read_cr2()))
                write_cr2(this_cpu_read(nmi_cr2));

so that it normally only does a read. Only if you actually took a page
fault will it restore cr2 to the old value (and if you took a page
fault the performance issues will be _there_, not in the "restore cr2"
part)

                Linus