Re: [Xen-devel] [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform changes

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 1/9/07 14:31, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

>> There's only one 'ACPI interrupt line' though, and presumably the re-eval
>> happens in dom0 somewhere, sometime later, and possibly on a
>> different CPU
>> from the one that took the interrupt. What happens if you try and enter a
>> C
>> state that is no longer available?
>> 
>> -- Keir
> 
> Yes, this is even true on native. SCI happens on one CPU with another
> CPU is decided to enter some unavailable C-state at the point. I guess
> hardware should tolerate such invalid request like taking it as a no-op
> or choosing a closest one. Software can anyway issue an invalid request
> to break report from hardware...

While I'm hassling you about C states: are there any examples in real system
of available C states changing dynamically? I mean, it makes sense to me
that available power/frequency combinations may change in response to
something like AC power being removed (this may make higher power/freq
options unavailable). But why would available C states change? I thought
these states were implemented internally on the CPU, and so are either
universally available or not, and I don't see any physical explanation for
why available deep-sleep options would be affected by e.g., battery vs. AC
operation.

 -- Keir



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