linux-kernel - Re: [PATCH] KVM: x86: synthesize TSA CPUID bits via SCATTERED

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Message-ID: <20260209174559.GDaYodVxWsiesiedLJ@fat_crate.local>
Date: Mon, 9 Feb 2026 18:45:59 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Carlos López <clopez@...e.de>,
	Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>,
	Babu Moger <bmoger@....com>
Subject: Re: [PATCH] KVM: x86: synthesize TSA CPUID bits via SCATTERED_F()

On Mon, Feb 09, 2026 at 08:29:36AM -0800, Sean Christopherson wrote:
> Nope.  KVM cares about what KVM can virtualize/emulate, and about helping userspace
> accurately represent the virtual CPU that will be enumerated to the guest.

So why don't you key on that in those macros instead of how they're defined?

	EXPOSE_TO_GUEST_F()

and then underneath we can figure out how to expose them.

We could have a helper table which determines what each feature is and how it
should interact with raw host CPUID or something slicker.

>   F               : Features that must be present in boot_cpu_data and raw CPUID
>   SCATTERED_F     : Same as F(), but are scattered by the kernel
>   X86_64_F        : Same as F(), but are restricted to 64-bit kernels
>   EMULATED_F      : Always supported; the feature is unconditionally emulated in software
>   SYNTHESIZED_F   : Features that must be present in boot_cpu_data, but may or
>                     may not be in raw CPUID.  May also be scattered.
>   PASSTHROUGH_F   : Features that must be present in raw CPUID, but may or may
>                     not be present in boot_cpu_data
>   ALIASED_1_EDX_F : Features in 0x8000_0001.EDX that are duplicates of identical 0x1.EDX features
>   VENDOR_F        : Features that are controlled by vendor code, often because
>                     they are guarded by a vendor specific module param.  Rules
>                     vary, but typically they are handled like basic F() features
>   RUNTIME_F       : Features that KVM dynamically sets/clears at runtime, but that
>                     are never adveristed to userspace.  E.g. OSXSAVE and OSPKE.

And for the time being, I'd love if this were somewhere in
arch/x86/kvm/cpuid.c so that it is clear how one should use those macros.

The end goal of having the user not care about which macro to use would be the
ultimate, super-duper thing tho.

I'd say.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette