Re: [PATCH 2/3] acpi/processor: sanitize _PDC buffer bits when running as Xen dom0

[Jason Andryuk <jandryuk@xxxxxxxxx>]

Hi, Roger,

On Mon, Nov 21, 2022 at 10:04 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> On Mon, Nov 21, 2022 at 03:10:36PM +0100, Jan Beulich wrote:
> > On 21.11.2022 11:21, Roger Pau Monne wrote:
> > > --- a/drivers/acpi/processor_pdc.c
> > > +++ b/drivers/acpi/processor_pdc.c
> > > @@ -137,6 +137,14 @@ acpi_processor_eval_pdc(acpi_handle handle, struct 
> > > acpi_object_list *pdc_in)
> > >             buffer[2] &= ~(ACPI_PDC_C_C2C3_FFH | ACPI_PDC_C_C1_FFH);
> > >
> > >     }
> > > +   if (xen_initial_domain())
> > > +           /*
> > > +            * When Linux is running as Xen dom0 it's the hypervisor the
> > > +            * entity in charge of the processor power management, and so
> > > +            * Xen needs to check the OS capabilities reported in the _PDC
> > > +            * buffer matches what the hypervisor driver supports.
> > > +            */
> > > +           xen_sanitize_pdc((uint32_t *)pdc_in->pointer->buffer.pointer);
> > >     status = acpi_evaluate_object(handle, "_PDC", pdc_in, NULL);
> >
> > Again looking at our old XenoLinux forward port we had this inside the
> > earlier if(), as an _alternative_ to the &= (I don't think it's valid
> > to apply both the kernel's and Xen's adjustments). That would also let
> > you use "buffer" rather than re-calculating it via yet another (risky
> > from an abstract pov) cast.
>
> Hm, I've wondered this and decided it wasn't worth to short-circuit
> the boot_option_idle_override conditional because ACPI_PDC_C_C2C3_FFH
> and ACPI_PDC_C_C1_FFH will be set anyway by Xen in
> arch_acpi_set_pdc_bits() as part of ACPI_PDC_C_CAPABILITY_SMP.
>
> I could re-use some of the code in there, but didn't want to make it
> more difficult to read just for the benefit of reusing buffer.
>
> > It was the very nature of requiring Xen-specific conditionals which I
> > understand was the reason why so far no attempt was made to get this
> > (incl the corresponding logic for patch 1) into any upstream kernel.
>
> Yes, well, it's all kind of ugly.  Hence my suggestion to simply avoid
> doing any ACPI Processor object handling in Linux with the native code
> and handle it all in a Xen specific driver.  That requires the Xen
> driver being able to fetch more data itself form the ACPI Processor
> methods, but also unties it from the dependency on the data being
> filled by the generic code, and the 'tricks' is plays into fooling
> generic code to think certain processors are online.

Are you working on this patch anymore?  My Xen HWP patches need a
Linux patch like this one to set bit 12 in the PDC.  I had an affected
user test with this patch and it worked, serving as an equivalent of
Linux commit a21211672c9a ("ACPI / processor: Request native thermal
interrupt handling via _OSC").

Another idea is to use Linux's arch_acpi_set_pdc_bits() to make the
hypercall to Xen.  It occurs earlier:
acpi_processor_set_pdc()
    acpi_processor_alloc_pdc()
        acpi_set_pdc_bits()
            arch_acpi_set_pdc_bits()
    acpi_processor_eval_pdc()

So the IDLE_NOMWAIT masking in acpi_processor_eval_pdc() would still
apply.  arch_acpi_set_pdc_bits() is provided the buffer, so it's a
little cleaner in that respect.

Thanks,
Jason