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Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops

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On Mon, Sep 21, 2015 at 9:49 AM, Arjan van de Ven <arjan@linux.intel.com> wrote:
> 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.

And maybe the KVM user return notifier.  Unfortunately, switch_to
might gain another two MSR accesses at some point if we decide to fix
the bugs in there.  Sigh.

>
> 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.
>

I tend to agree.

Anyway, the fully out-of-line approach isn't obviously a bad idea, and
it simplifies the whole mess (we can drop most of the paravirt
patches, too).  I'll give it a try and see what happens.

--Andy
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[PATCH v2 0/2] x86/msr: MSR access failure changes Andy Lutomirski <luto@kernel.org> - 2015-09-21 02:10 +0200
  [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops Andy Lutomirski <luto@kernel.org> - 2015-09-21 02:10 +0200
    Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-21 02:20 +0200
      Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Andy Lutomirski <luto@amacapital.net> - 2015-09-21 03:20 +0200
        Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Ingo Molnar <mingo@kernel.org> - 2015-09-21 10:50 +0200
          Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Paolo Bonzini <pbonzini@redhat.com> - 2015-09-21 14:30 +0200
          Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-21 18:40 +0200
            Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Arjan van de Ven <arjan@linux.intel.com> - 2015-09-21 19:10 +0200
              Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-21 19:30 +0200
              Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Andy Lutomirski <luto@amacapital.net> - 2015-09-21 19:50 +0200
                Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Paolo Bonzini <pbonzini@redhat.com> - 2015-09-22 10:20 +0200
            Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Andy Lutomirski <luto@amacapital.net> - 2015-09-21 20:20 +0200
              Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Borislav Petkov <bp@alien8.de> - 2015-09-21 20:40 +0200
              Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-21 20:50 +0200
            Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Ingo Molnar <mingo@kernel.org> - 2015-09-22 09:20 +0200
            Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Peter Zijlstra <peterz@infradead.org> - 2015-09-30 15:20 +0200
              Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Ingo Molnar <mingo@kernel.org> - 2015-09-30 16:10 +0200
                Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Andy Lutomirski <luto@amacapital.net> - 2015-09-30 20:10 +0200
                Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops Ingo Molnar <mingo@kernel.org> - 2015-10-01 09:20 +0200
            Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access  fails without !panic_on_oops "H. Peter Anvin" <hpa@zytor.com> - 2015-09-30 20:40 +0200
  [PATCH v2 2/2] x86/msr: Set the return value to zero when native_rdmsr_safe fails Andy Lutomirski <luto@kernel.org> - 2015-09-21 02:10 +0200

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