linux-kernel - Re: [PATCH v2] x86/bhyve: Detect FreeBSD Bhyve hypervisor

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Message-ID: <aKL0WlA4wIU8l9RT@lx-t490>
Date: Mon, 18 Aug 2025 11:37:30 +0200
From: "Ahmed S. Darwish" <darwi@...utronix.de>
To: David Woodhouse <dwmw2@...radead.org>
Cc: 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>, linux-kernel@...r.kernel.org,
	Konstantin Belousov <kib@....kiev.ua>,
	John Baldwin <jhb@...ebsd.org>,
	"<cperciva@...snap.com>" <cperciva@...snap.com>
Subject: Re: [PATCH v2] x86/bhyve: Detect FreeBSD Bhyve hypervisor

On Fri, 15 Aug 2025, David Woodhouse wrote:
>
> Or just not, in the case of jailhouse and bhyve? Since cpuid_feature()
> is hard-coded to true for x86_64 anyway, and they both depend on that
> (like acrn, which already only checks for X86_FEATURE_HYPERVISOR).
>
> And anyway, how is X86_FEATURE_HYPERVISOR even going to get set if
> there's no CPUID? Can that happen?
>

Correct, these early-boot feature detection code paths are all already
messed up.  I will clean them as part of the CPUID parser work.

For example, in the CPUID parser patch queue next iteration (v5),
X86_FEATURE flags are integrated.  There's also a Directed Acylic Graph
of X86_FEATURE and CPUID bits dependencies.

So, in the above case, the dependencies table will just have:

    X86_FEATURE_HYPERVISOR ► X86_FEATURE_CPUID (and whatever else)

and all the circles of x86 vendor checking, CPUID max level checking,
other CPUID-bits checking, X86_FEATURE dependency checking, etc. will all
be removed from call sites.

I will show more of that at LPC Japan x86 track, and hopefully much
before that the CPUID parser v5 shall also be already posted to LKML.

Thanks!
Ahmed