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

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Message-ID: <CALCETrVMV2_3ywQ_t+0rsqzxMm6D9PvDEmOdrie67rRzfj-W_Q@mail.gmail.com>
Date:	Wed, 30 Sep 2015 11:04:29 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KVM list <kvm@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] x86/msr: Carry on after a non-"safe" MSR access
 fails without !panic_on_oops

On Wed, Sep 30, 2015 at 7:01 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Mon, Sep 21, 2015 at 09:36:15AM -0700, Linus Torvalds wrote:
>> > On Mon, Sep 21, 2015 at 1:46 AM, Ingo Molnar <mingo@...nel.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? Get rid of the inline version of the _safe()
>> > thing too, and put that thing there too.
>>
>> There are a few in the perf code, and esp. on cores without a stack engine the
>> call overhead is noticeable. Also note that the perf MSRs are generally
>> optimized MSRs and less slow (we cannot say fast, they're still MSRs) than
>> regular MSRs.
>
> These could still be open coded in an inlined fashion, like the scheduler usage.
>

We could have a raw_rdmsr for those.

OTOH, I'm still not 100% convinced that this warn-but-don't-die
behavior is worth the effort.  This isn't a frequent source of bugs to
my knowledge, and we don't try to recover from incorrect cr writes,
out-of-bounds MMIO, etc, so do we really gain much by rigging a
recovery mechanism for rdmsr and wrmsr failures for code that doesn't
use the _safe variants?

--Andy

> Thanks,
>
>         Ingo



-- 
Andy Lutomirski
AMA Capital Management, LLC
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