linux-kernel - Re: [PATCH] x86/mm: Check cc_vendor when printing memory encryption info

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Date:   Fri, 10 Nov 2023 17:45:57 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Michael Kelley <mhklinux@...look.com>,
        Dexuan Cui <decui@...rosoft.com>, linux-hyperv@...r.kernel.org,
        stefan.bader@...onical.com, tim.gardner@...onical.com,
        roxana.nicolescu@...onical.com, cascardo@...onical.com,
        kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        kirill.shutemov@...ux.intel.com, sashal@...nel.org
Subject: Re: [PATCH] x86/mm: Check cc_vendor when printing memory encryption
 info

On Fri, Nov 10, 2023 at 04:51:43PM +0100, Jeremi Piotrowski wrote:
> What's semi-correct about checking for CC_VENDOR_INTEL and then
> printing Intel?  I can post a v2 that checks CC_ATTR_GUEST_MEM_ENCRYPT
> before printing "TDX".

How is it that you're not seeing the conflict:

Your TD partitioning guest *is* a TDX guest so X86_FEATURE_TDX_GUEST
should be set there. But it isn't. Which means, that is already wrong.
Or insufficient.

	 if (cc_vendor == CC_VENDOR_INTEL)

just *happens* to work for your case.

What the detection code should do, rather, is:

	if (guest type == TD partioning)
		set bla;
	else if (TDX_CPUID_LEAF_ID)
		"normal" TDX guest;

and those rules need to be spelled out so that everyone is on the same
page as to how a TD partitioning guest is detected, how a normal TDX
guest is detected, a SEV-ES, a SNP one, yadda yadda.

> The paravisor *is* telling the guest it is running on one - using a CPUID leaf
> (HYPERV_CPUID_ISOLATION_CONFIG). A paravisor is a hypervisor for a confidential
> guest, that's why paravisor detection shares logic with hypervisor detection.
> 
> tdx_early_init() runs extremely early, way before hypervisor(/paravisor) detection.

What?

Why can't tdx_early_init() run CPUID(HYPERV_CPUID_ISOLATION_CONFIG) if
it can't find a valid TDX_CPUID_LEAF_ID and set X86_FEATURE_TDX_GUEST
then?

> Additionally we'd need to sprinkle paravisor checks along with
> existing X86_FEATURE_TDX_GUEST checks. And any time someone adds a new
> feature that depends solely on X86_FEATURE_TDX_GUEST we'd run the
> chance of it breaking things.

Well, before anything, you'd need to define what exactly the guest kernel
needs to do when running as a TD partitioning guest and how exactly that
is going to be detected and checked using the current cc_* and
cpufeatures infra. If it doesn't work with the current scheme, then the
current scheme should be extended.

Then, that should be properly written out:

"if bit X is set, then that is a guest type Y"
"if feature foo present, then so and so are given"

If the current guest type detection is insufficient, then that should be
extended/amended.

That's the only viable way where the kernel would support properly and
reliably a given guest type. There'll be no sprinkling of checks
anywhere.

Thx.

-- 
Regards/Gruss,
    Boris.

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