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

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Date: Wed, 24 Apr 2024 20:46:40 +0200
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, Apr 04, 2024 at 07:07:26PM +0200, Jeremi Piotrowski wrote:
> On 28/03/2024 16:39, Borislav Petkov wrote:
> > 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?
> > 
> 
> No problem now but I did find it odd that cc_vendor will now always be set for AMD but
> not for Intel. For Intel the various checks would automatically return true. Something
> to look out for in the future when adding CC_ATTR's - no one can assume that the checks
> will only run when actively dealing with confidential computing.

Right, I haven't made up my mind fully here yet... setting cc_vendor
*only* when running as some sort of a confidential computing guest kinda
makes sense.

And if it is not set, then that can be used to catch cases where the
cc_* helpers are used outside of confidential computing cases...

Do we want those assertions? I don't know...

> I see your point about the disable needing to happen late - but then how about we remove
> the setup_clear_cpu_cap(X86_FEATURE_SEV_SNP) too? No code depends on it any more and it would
> help my cause as well.
> 
> > So we need a test for "am I a nested SNP hypervisor?"
> > 
> > So, can your thing clear X86_FEATURE_HYPERVISOR and thus "emulate"
> > baremetal?
> > 
> 
> Can't do that... it is a VM and hypervisor detection and various paravirt interfaces depend on
> X86_FEATURE_HYPERVISOR.

Right, but "your cause" as you call it above looks like a constant
whack'a'mole game everytime we change something in the kernel when
enabling those things and that breaks your cause.

Do you really want that?

Or would you prefer to define your nested solution properly and then
have upstream code support it like the next well-defined coco platform
instead?

Thx.

-- 
Regards/Gruss,
    Boris.

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