RE: [Xen-devel] [PATCH]ACPI: re-enable mwait for xen cpuidle

["Wei, Gang" <gang.wei@xxxxxxxxx>]

On Saturday, 2010-4-3 2:33 AM, Jeremy Fitzhardinge wrote:
> On 04/02/2010 09:27 AM, Wei, Gang wrote:
>> Updated the 2nd patch, only set MWAIT feature for dom0.
>> 
>> Jimmy
>> 
>> ACPI: re-enable mwait for xen cpuidle
>> 
>> Xen hypervisor doesn't export mwait feature to dom0, but latest
>> Linux kernel start to check this feature while initializing _PDC
>> object. Modify xen_cpuid to re-enable mwait for xen cpuidle.  
>> 
> 
> What if the CPU really doesn't have MWAIT?

If the CPU really doesn't have MWAIT, BIOS should know it and BIOS acpi code 
should not give C state table with MWAIT entry method. Even the BIOS give the 
wrong MWAIT C state info, xen cpuidle will refuse it and mark that C state as 
invalid.

> But I agree with your original assessment that setting MWAIT just to
> get a couple of paths in ACPI parsing enabled is probably overkill,
> but I don't like the idea of putting xen-specific tests into the acpi
> code. 

I don't like it too. But some time we have to accept a workaround temporarily 
even we don't like it, until we find a graceful solution.

> Would it be possible to change the parser code to parse
> unconditionally and then ignore the MWAIT-specific stuff later on? 
> (I haven't looked at the structure of the code, so I'm not sure if
> this suggestion even makes sense.)

That means to turn back to old change set. In 2.6.18, this check doesn't exist 
in the parser code path. I have to say, these checks made Linux kernel itself 
more robust. I am not sure whether we can find a better way which is also 
compatible with Xen's need.

Jimmy


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