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

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Message-ID: <aYoLcPkjJChCQM7E@google.com>
Date: Mon, 9 Feb 2026 08:29:36 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Borislav Petkov <bp@...en8.de>
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, Borislav Petkov wrote:
> On Mon, Feb 09, 2026 at 07:06:36AM -0800, Sean Christopherson wrote:
> > Maybe I could add a table showing how the XXX_F() macros map to various controls?
> 
> Perhaps start first, please, with explaining the difference between
> SYNTHESIZED_F and SCATTERED_F and why you even need it?

SCATTERED_F requires the feature flag to be present in raw CPUID, SYNTHESIZED_F
does not.

> I mean, KVM and guests only care whether X86_FEATURE flags are set or not, no?

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.

  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.

> Not how they get defined underneath...