Re: [Xen-devel] expose MWAIT to dom0

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> 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.

> It's unlikely to make into upstream, and also get lost in
> into some distro such as SLES11.

We can certainly fix it there.

Jan


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