linux-kernel - Re: [PATCH] EDAC/mce_amd: Reduce unnecessary spew in dmesg if SMCA feature bit is not exposed

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Date:   Tue, 15 Jun 2021 19:25:57 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Yazen Ghannam <yazen.ghannam@....com>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        James Morse <james.morse@....com>,
        Robert Richter <rric@...nel.org>
Subject: Re: [PATCH] EDAC/mce_amd: Reduce unnecessary spew in dmesg if SMCA
 feature bit is not exposed

On Tue, Jun 15, 2021 at 12:32:21PM -0400, Yazen Ghannam wrote:
> Yes, I agree. I was a bit confused about the X86_FEATURE_HYPERVISOR
> thing, but I think I get it. This definitely looks simple to do.

Yeah, if you look at the qemu repo:

kvm_arch_get_supported_cpuid:

...

    } else if (function == 1 && reg == R_ECX) {
        /* We can set the hypervisor flag, even if KVM does not return it on
         * GET_SUPPORTED_CPUID
         */
        ret |= CPUID_EXT_HYPERVISOR;


so long story short, that thing is set by the HV and there's an
"agreement" of sorts and APM v3 even says so:

"CPUID Fn0000_0001_ECX Feature Identifiers

...

31:	RAZ. Reserved for use by hypervisor to indicate guest status."

HTH.

-- 
Regards/Gruss,
    Boris.

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