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Shutemov" , "Edgecombe, Rick P" , Andrew Cooper Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit First, Jon and John gave a talk in Tokyo about feature enumeration under paravisors: > https://lpc.events/event/19/contributions/2188/attachments/1896/4057/05-Paravisor-Integration-with-Confidential-Services.pdf The tl;dr for me at least was that they'd like a common and consistent means of enumerating these features in OSes, regardless of the environment: TDX, SEV-SNP or even ARM CCA. I wanted to explore one corner of the solution space a bit. 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. 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. 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? Is there anything stopping us from carving out a chunk of CPUID for this purpose?