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Re: [Xen-devel] [PATCH v4 11/14] pnpbios: replace paravirt_enabled() check with legacy device check

On Thu, Apr 07, 2016 at 10:46:11AM +0100, David Vrabel wrote:
> On 07/04/16 01:06, Luis R. Rodriguez wrote:
> > Since we are removing paravirt_enabled() replace it with a
> > logical equivalent. Even though PNPBIOS is x86 specific we
> > add an arch-specific type call, which can be implemented by
> > any architecture to show how other legacy attribute devices
> > can later be also checked for with other ACPI legacy attribute
> > flags.
> > 
> > This implicates the first ACPI IA-PC Boot Architecture
> > ACPI_FADT_LEGACY_DEVICES flag device, and shows how to add more.
> [...]
> > +struct x86_legacy_devices {
> > +   int pnpbios;
> > +};
> It's not clear why pnpbios needs a new structure

I'm glad you asked. Dealing with placing pnpbios quirk in a more useful generic
fashion was perhaps the most difficult challenge in this series.  As I reviewed
possibilities to remove paravirt_enabled() the best prospect I found was to see
if Xen could instead use ACPI IA-PC Boot Architecture flags to annotate
some quirks. It turns out that it is possible, but there are only so many flags,
and also, we didn't want to have a solution that incurred respective upstream
Xen hypervisor change, that would be silly.

To make this quirk more useful then this folds the pnpbios quirk as a sub
quirk under the more borad ACPI_FADT_LEGACY_DEVICES ACPI flag. What this
does, as can be seen by also looking at the next patch, "x86, ACPI: parse
ACPI_FADT_LEGACY_DEVICES" is it explicitly folds pnpbios as one of the
ACPI_FADT_LEGACY_DEVICES devices, but also paves the way for further known
main legacy components to added to the list.

> and why this structure of devices does not have the bit for the rtc device.

That's because ACPI has its own dedicated flag for it, so there already
is a one-to-one mapping available. All we needed to do to replace the
RTC hack was to provide a mechanism to unify both the paravirt RTC hack
with the ACPI RTC flag.


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