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SubjectRe: [PATCH v4 2/5] x86/umwait: Initialize umwait control values
On Mon, 17 Jun 2019, Fenghua Yu wrote:
> On Tue, Jun 11, 2019 at 10:46:55PM +0200, Thomas Gleixner wrote:
> > On Sun, 9 Jun 2019, Fenghua Yu wrote:
> > > > Sounds good, but:
> > > >
> > > > > +#define MSR_IA32_UMWAIT_CONTROL_C02 BIT(0)
> > > >
> > > > > +static u32 umwait_control_cached = 100000;
> > > >
> > > > The code seems to disagree.
> > >
> > > The definition of bit[0] is: C0.2 is disabled when bit[0]=1. So
> > > 100000 means C0.2 is enabled (and max time is 100000).
> >
> > which is totally non obvious. If you have to encode the control bit, then
> > please make it explicit, i.e. mask out the disable bit in the initializer.
>
> Is this right?
>
> static u32 umwait_control_cached = 100000 & ~MSR_IA32_UMWAIT_CONTROL_C02_DISABLED;

Works, but looks pretty odd. I'd rather create an explicit initializer
macro, something like:

UMWAIT_CTRL_VAL(100000, UMWAIT_DISABLED);

Hmm?

Thanks,

tglx

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