On Wed, Sep 30, 2015 at 7:01 AM, Ingo Molnar <mi...@kernel.org> wrote:
>
> * Peter Zijlstra <pet...@infradead.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 <mi...@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? 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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