linux-kernel - Re: [PATCH 2/4] KVM: x86: Advertise AMX CPUIDs in subleaf 0x1E.0x1 to userspace

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Message-ID: <aXO1lhrCb-y8oEnF@google.com>
Date: Fri, 23 Jan 2026 09:53:26 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Zhao Liu <zhao1.liu@...el.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Chao Gao <chao.gao@...el.com>, 
	Xudong Hao <xudong.hao@...el.com>
Subject: Re: [PATCH 2/4] KVM: x86: Advertise AMX CPUIDs in subleaf 0x1E.0x1 to userspace

On Fri, Jan 23, 2026, Xiaoyao Li wrote:
> On 11/20/2025 1:07 PM, Zhao Liu wrote:
> > In addition to the new features, CPUID 0x1E.0x1.EAX[bits 0-3] are
> > mirrored positions of existing AMX feature bits distributed across the
> > 0x7 leaves. To avoid duplicate feature names, name these mirror bits
> > with a *_MIRROR suffix, and define them in reverse_cpuid.h as KVM-only
> > features as well.
> 
> It looks that KVM can emulate the mirroring CPUIDs regardless of whether
> hardware supports subleaf 1. However, given such emulation provides no real
> benefit but complicates KVM implementation, this patch looks good to me.

Yeah, and it would run the risk of guest software doing stupid things like
assuming a certain CPU generation if the leaf is supported.