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 15:24:29 +0100
From: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To: Borislav Petkov <bp@...en8.de>
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 28/03/2024 14:41, Borislav Petkov wrote:
> On Thu, Mar 28, 2024 at 12:51:17PM +0100, Jeremi Piotrowski wrote:
>> Shouldn't this line be inside the cpu_has(c, X86_FEATURE_SEV_SNP) check?
> 
> The cc_vendor is not dependent on X86_FEATURE_SEV_SNP.
>

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.

cc_vendor being unset is handled correctly in cc_platform_has() checks.

>> How about turning this into a more specific check:
>>
>>   if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP) &&
> 
> Why?
> 

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

> The check is "am I running as a hypervisor on baremetal".
> 

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

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