Re: [Xen-devel] [PATCH] x86/mwait_idle: Allow setting the max cstate to C1

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.06.14 at 16:43, <ross.lagerwall@xxxxxxxxxx> wrote:
> From: Ross Lagerwall <rosslagerwall@xxxxxxxxxx>
> 
> Following 91413b519631 ("x86/mwait_idle: export both C1 and C1E"), when
> setting the max cstate to C1, the C1E cstate is used as well. This is
> because MWAIT_HINT2CSTATE returns the same value for C1 and C1E.
> Instead, when limiting the cstate, compare max_cstate with the position
> in the states array, as the acpi cpu_idle driver does.
> 
> Without this patch, there's no way of setting the max cstate to C1 when using
> the mwait_idle driver.

But it was intentionally this way from the beginning of the existence of
the mwait idle driver - the other approach makes the value to be passed
really platform dependent (i.e. "max_cstate=2" doesn't universally mean
what one would expect: maximum C-state is C2).

But I recognize the need to disable all possible (intermediate) levels; I
just think this should be done via extending max_cstate= (e.g. by
allowing for a second number, which would then indicate the maximum
sub-state).

Jan


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