linux-kernel - Re: [PATCH 5/5] x86/CPU/AMD: Track SNP host status with cc_platform

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Date: Thu, 28 Mar 2024 16:39:14 +0100
From: Borislav Petkov <bp@...en8.de>
To: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>, Ashish Kalra <ashish.kalra@....com>,
	Joerg Roedel <joro@...tes.org>, Michael Roth <michael.roth@....com>,
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 5/5] x86/CPU/AMD: Track SNP host status with
 cc_platform_*()

On Thu, Mar 28, 2024 at 03:24:29PM +0100, Jeremi Piotrowski wrote:
> It's not but if you set it before the check it will be set for all AMD
> systems, even if they are neither CC hosts nor CC guests.

That a problem?

It is under a CONFIG_ARCH_HAS_CC_PLATFORM...

> To leave open the possibility of an SNP hypervisor running nested.

But !CC_ATTR_GUEST_SEV_SNP doesn't mean that. It means it is not
a SEV-SNP guest.

> I thought you wanted to filter out SEV-SNP guests, which also have
> X86_FEATURE_SEV_SNP CPUID bit set.

I want to run snp_probe_rmptable_info() only on baremetal where it makes
sense.

> My understanding is that these are the cases:
> 
> CPUID(SEV_SNP) | MSR(SEV_SNP)     | what am I
> ---------------------------------------------
> set            | set              | SNP-guest
> set            | unset            | SNP-host
> unset          | ??               | not SNP

So as you can see, we can't use X86_FEATURE_SEV_SNP for anything due to
the late disable need. So we should be moving away from it.

So we need a test for "am I a nested SNP hypervisor?"

So, can your thing clear X86_FEATURE_HYPERVISOR and thus "emulate"
baremetal?

-- 
Regards/Gruss,
    Boris.

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