linux-kernel - Re: "Paravisor" Feature Enumeration

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Message-ID: <71f42b9e-793b-4f8d-8159-a6ca7800f292@intel.com>
Date: Mon, 5 Jan 2026 16:36:25 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: dan.j.williams@...el.com, "Lange, Jon" <jlange@...rosoft.com>
Cc: 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

On 1/5/26 16:01, dan.j.williams@...el.com wrote:
> Dave Hansen wrote:
...
>> 	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?

Yes.

...
>> 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.

The way it _seems_ to have worked until now is that KVM/x86 has led the
way by defining a CPUID bit for things like KVM_CLOCK of KVM_STEAL_TIME.
Then, the ARM folks came along and DeviceTree enumerations. Last, ACPI
came along with a way to package up all the DeviceTree enumerations into
a single table.

So, maybe that's a hack on a hack on a hack and we should just start
with ACPI this time. That would certainly make this pretty straightforward.

I'd love to hear a take from the x86/KVM folks, though.