linux-kernel - Re: [PATCH AUTOSEL 5.10 2/6] x86/cpu: Don't clear X86_FEATURE_LAHF_LM flag in init_amd_k8() on AMD when running in a virtual machine

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Message-ID: <aAKJkrQxp5on46nC@google.com>
Date: Fri, 18 Apr 2025 10:19:14 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Pavel Machek <pavel@...x.de>
Cc: Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
	Max Grobecker <max@...becker.info>, Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com, 
	x86@...nel.org, thomas.lendacky@....com, perry.yuan@....com, 
	mario.limonciello@....com, riel@...riel.com, mjguzik@...il.com, 
	darwi@...utronix.de
Subject: Re: [PATCH AUTOSEL 5.10 2/6] x86/cpu: Don't clear X86_FEATURE_LAHF_LM
 flag in init_amd_k8() on AMD when running in a virtual machine

On Fri, Apr 18, 2025, Pavel Machek wrote:
> Hi!
> 
> > From: Max Grobecker <max@...becker.info>
> > 
> > [ Upstream commit a4248ee16f411ac1ea7dfab228a6659b111e3d65 ]
> 
> > This can prevent some docker containers from starting or build scripts to create
> > unoptimized binaries.
> > 
> > Admittably, this is more a small inconvenience than a severe bug in the kernel
> > and the shoddy scripts that rely on parsing /proc/cpuinfo
> > should be fixed instead.

Uh, and the hypervisor too?  Why is the hypervisor enumerating an old K8 CPU for
what appears to be a modern workload?

> I'd say this is not good stable candidate.

Eh, practically speaking, there's no chance of this causing problems.  The setup
is all kinds of weird, but AIUI, K8 CPUs don't support virtualization so there's
no chance that the underlying CPU is actually affected by the K8 bug, because the
underlying CPU can't be K8.  And no bare metal CPU will ever set the HYPERVISOR
bit, so there won't be false positives on that front.

I personally object to the patch itself; it's not the kernel's responsibility to
deal with a misconfigured VM.  But unless we revert the commit, I don't see any
reason to withhold this from stable@.