* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Sep 21, 2015 at 1:46 AM, Ingo Molnar <mingo@xxxxxxxxxx> 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.
Absolutely!
> 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.
Only a very low number of them is performance critical (because even
hw-accelerated MSR accesses are generally slow so we try to avoid MSR accesses in
fast paths as much as possible, via shadowing, etc.) - and in the few cases where
we have to access an MSR in a fast path we can do those separately.
I'm only worried about the 'default' APIs, i.e. rdmsr() that is used throughout
arch/x86/ over a hundred times, not about performance critical code paths that get
enough testing and enough attention in general.
Thanks,
Ingo
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