linux-kernel - Re: "Paravisor" Feature Enumeration

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Message-ID: <695c50eee68e3_4b7a100e8@dwillia2-mobl4.notmuch>
Date: Mon, 5 Jan 2026 16:01:50 -0800
From: <dan.j.williams@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, "Lange, Jon" <jlange@...rosoft.com>
CC: "Williams, Dan J" <dan.j.williams@...el.com>, Sean Christopherson
	<seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
	<john.starks@...rosoft.com>, Will Deacon <will@...nel.org>, Mark Rutland
	<mark.rutland@....com>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, LKML <linux-kernel@...r.kernel.org>, "Kirill A.
 Shutemov" <kirill.shutemov@...ux.intel.com>, "Edgecombe, Rick P"
	<rick.p.edgecombe@...el.com>, Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: "Paravisor" Feature Enumeration

Dave Hansen wrote:
> 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.

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.