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There was a > pretty limited audience of folks in the room. Please feel free to flesh > out the cc list with anyone I missed. > > Dan Williams' first thought seemed to revolve around having some kind of > platform-independent device that could do the enumeration. Maybe a > synthetic PCI device. I'm sure Dan can chime in to fill in the details > that I missed. More that it sounded like "just another firmware enumeration" problem, where a platform device is one of the results along with related firmware tables and objects. > I immediately just thought of CPUID. We already have a whole region of > CPUID (0x40000000) that hypervisors use to enumerate stuff to guests by > convention. It wouldn't be a large leap at all to carve out a chunk of > that so that paravisors can use it. > > But the biggest barrier I see there is that our ARM friends don't have > CPUID. It seems like they _mostly_ have bit-by-bit aliases in ACPI or > DeviceTree for the x86 CPUID bits, like: > > X86_FEATURE_KVM_CLOCKSOURCE in arm,pvclock > or > X86_FEATURE_KVM_STEAL_TIME in arm,kvm-steal-time > > As far as I can tell, these aliases are all done ad-hoc. This approach > could obviously be extended to paravisor features, but it would probably > be on the slow side to do it for each new feature. "Slow" as in standardization time? > It _seems_ like we could pick a chunk of CPUID space (say 32-bits of it) > and alias it 1:1 with some DeviceTree/ACPI property, say > "arm,paravisor-features". Kernel code would just be written to say > "check feature 13" and the arch-specific helpers would either steer that > to CPUID or DeviceTree. > > Is there anything like that today that's cross-architecture and > cross-hypervisor? That seems the definition of an ACPI description. > Is there anything stopping us from carving out a chunk of CPUID for > this purpose? At what point does an ACPI property become a CPUID? In other words if there is an ACPI / DeviceTree enumeration of CPU/platform capabilities in firmware that can supsersede / extend native enumeration, does it matter if x86 maps that to extended CPUID space and ARM maps it however is convenient? I have no problem with an extended CPUID concept, just trying to understand more about the assumptions.