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Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>
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Cc: Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paolo Bonzini <pbonzini@redhat.com>,
        xen-devel <Xen-devel@lists.xen.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
From: Arjan van de Ven <arjan@linux.intel.com>
Message-ID: <56003506.1050100@linux.intel.com>
Date: Mon, 21 Sep 2015 09:49:10 -0700
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On 9/21/2015 9:36 AM, Linus Torvalds wrote:
> On Mon, Sep 21, 2015 at 1:46 AM, Ingo Molnar <mingo@kernel.org> wrote:
>>
>> Linus, what's your preference?
>
> So quite frankly, is there any reason we don't just implement
> native_read_msr() as just
>
>     unsigned long long native_read_msr(unsigned int msr)
>     {
>        int err;
>        unsigned long long val;
>
>        val = native_read_msr_safe(msr, &err);
>        WARN_ON_ONCE(err);
>        return val;
>     }
>
> Note: no inline, no nothing. Just put it in arch/x86/lib/msr.c, and be
> done with it. I don't see the downside.
>
> How many msr reads are <i>so</i> critical that the function call
> overhead would matter?

if anything qualifies it'd be switch_to() and friends.

note that I'm not entirely happy about the notion of "safe" MSRs.
They're safe as in "won't fault".
Reading random MSRs isn't a generic safe operation though, but the name sort of gives people
the impression that it is. Even with _safe variants, you still need to KNOW the MSR exists (by means
of CPUID or similar) unfortunately.