RE: [Xen-devel] expose MWAIT to dom0

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 15.08.11 at 10:09, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx] 
>> Sent: Monday, August 15, 2011 4:02 PM
>> 
>> >>> On 15.08.11 at 07:35, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
>> > There're basically two methods to enter a given C-state: legacy (hlt + I/O
>> > read),
>> > and native(using mwait). MWAIT is always preferred when both underlying
>> CPU
>> > and OS support, which is a more efficient way to conduct C-state 
>> > transition.
>> >
>> > Xen PM relies on Dom0 to parse ACPI Cx/Px information, which involves one
>> > step to notify BIOS about a set of capabilities supported by OSPM. One
>> > capability
>> > is about mwait support, which if true ACPI Cx entry contains entry
>> > parameters
>> > for mwait, or else I/O port information is provided. Xen PM later decides
>> > entry
>> > method (i/o or mwait) based on parsed ACPI information from dom0.
>> >
>> > However Xen doesn't expose MWAIT capability to dom0 due to changeset
>> 17573:
>> >
>> > This then brings a problem to Dom0 which thinks underlying CPU
>> > doesn't report mwait, and thus notify BIOS to use old I/O based method.
>> >
>> > Later a trick is integrated in Jeremy's pvops tree:
>> >
>> > --- a/arch/x86/kernel/acpi/processor.c
>> > +++ b/arch/x86/kernel/acpi/processor.c
>> > @@ -60,7 +60,7 @@ static void init_intel_pdc(struct acpi_processor *pr,
>> > struct cpuinfo_x86 *c)
>> >         /*
>> >          * If mwait/monitor is unsupported, C2/C3_FFH will be disabled
>> >          */
>> > -       if (!cpu_has(c, X86_FEATURE_MWAIT))
>> > +       if (!cpu_has(c, X86_FEATURE_MWAIT) && !xen_initial_domain())
>> >                 buf[2] &= ~(ACPI_PDC_C_C2C3_FFH);
>> >
>> >         obj->type = ACPI_TYPE_BUFFER;
>> >
>> > Above trick is ugly and error-prone, since it always enable mwait 
>> > regardless
>> > of actual CPU capability.
>> 
>> 3.x (and later 2.6.3x) don't look at the CPUID flag anymore, they just
>> check boot_option_idle_override, which is being controlled from the
>> command line or enforced for some particular systems based on DMI
>> data.
> 
> I don't think so. "boot_option_idle_override" controls the way how idle loop
> is implemented, which has the side effect to disable MWAIT if cpuid says it
> but "boot=nomwait" is specified. But it has no effect to enable MWAIT if
> Xen doesn't tell dom0 about it. 
> 
> Check arch_acpi_set_pdc_bits under x86:

Indeed - I didn't look at the header files.

Jan


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