linux-kernel - Re: [PATCH v3] x86/sev: Add SEV-SNP guest feature negotiation support

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Message-ID: <Y7QwXcAUmS3VZcbH@zn.tnic>
Date:   Tue, 3 Jan 2023 14:40:45 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Nikunj A Dadhania <nikunj@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, dave.hansen@...ux.intel.com,
        seanjc@...gle.com, pbonzini@...hat.com, thomas.lendacky@....com,
        michael.roth@....com, stable@...nel.org
Subject: Re: [PATCH v3] x86/sev: Add SEV-SNP guest feature negotiation support

On Mon, Jan 02, 2023 at 02:08:10PM +0530, Nikunj A Dadhania wrote:
> The hypervisor can enable various new features (SEV_FEATURES[1:63])
> and start the SNP guest. Some of these features need guest side
> implementation. If any of these features are enabled without guest
> side implementation, the behavior of the SNP guest will be undefined.
> The SNP guest boot may fail in a non-obvious way making it difficult
> to debug.
> 
> Instead of allowing the guest to continue and have it fail randomly
> later, detect this early and fail gracefully.
> 
> SEV_STATUS MSR indicates features which hypervisor has enabled. While
					 ^
					 the

> booting, SNP guests should ascertain that all the enabled features
> have guest side implementation. In case any feature is not implemented
> in the guest, the guest terminates booting with SNP feature
> unsupported exit code.
> 
> More details in AMD64 APM[1] Vol 2: 15.34.10 SEV_STATUS MSR
> 
> [1] https://www.amd.com/system/files/TechDocs/40332_4.05.pdf
> 
> Fixes: cbd3d4f7c4e5 ("x86/sev: Check SEV-SNP features support")
> CC: Borislav Petkov <bp@...en8.de>
> CC: Michael Roth <michael.roth@....com>
> CC: Tom Lendacky <thomas.lendacky@....com>
> CC: <stable@...nel.org>
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>

...

> diff --git a/Documentation/x86/amd-memory-encryption.rst b/Documentation/x86/amd-memory-encryption.rst
> index a1940ebe7be5..b8b6b87be995 100644
> --- a/Documentation/x86/amd-memory-encryption.rst
> +++ b/Documentation/x86/amd-memory-encryption.rst
> @@ -95,3 +95,38 @@ by supplying mem_encrypt=on on the kernel command line.  However, if BIOS does
>  not enable SME, then Linux will not be able to activate memory encryption, even
>  if configured to do so by default or the mem_encrypt=on command line parameter
>  is specified.
> +
> +Secure Nested Paging (SNP):

No ":"

> +===========================

<---- newline here.

> +SEV-SNP introduces new features (SEV_FEATURES[1:63]) which can be enabled
> +by the hypervisor for security enhancements. Some of these features need
> +guest side implementation to function correctly. The below table lists the
> +expected guest behavior with various possible scenarios of guest/hypervisor
> +SNP feature support.
> +
> ++---------------+---------------+---------------+---------------+
> +|Feature Enabled|  Guest needs  |   Guest has   |  Guest boot   |
> +|     by HV     |implementation |implementation |   behavior    |
> ++---------------+---------------+---------------+---------------+
> +|      No       |      No       |      No       |     Boot      |
> +|               |               |               |               |
> ++---------------+---------------+---------------+---------------+
> +|      No       |      Yes      |      No       |     Boot      |
> +|               |               |               |               |
> ++---------------+---------------+---------------+---------------+
> +|      No       |      Yes      |      Yes      |     Boot      |
> +|               |               |               |               |
> ++---------------+---------------+---------------+---------------+
> +|      Yes      |      No       |      No       |   Boot with   |
> +|               |               |               |feature enabled|
> ++---------------+---------------+---------------+---------------+
> +|      Yes      |      Yes      |      No       | Graceful Boot |
> +|               |               |               |    Failure    |
> ++---------------+---------------+---------------+---------------+
> +|      Yes      |      Yes      |      Yes      |   Boot with   |
> +|               |               |               |feature enabled|
> ++---------------+---------------+---------------+---------------+

sphinx is not happy about that table for some reason. I always find the error
messages cryptic though:

Documentation/x86/amd-memory-encryption.rst:110: WARNING: Block quote ends without a blank line; unexpected unindent.
Documentation/x86/amd-memory-encryption.rst:110: WARNING: Block quote ends without a blank line; unexpected unindent.
Documentation/x86/amd-memory-encryption.rst:122: WARNING: Block quote ends without a blank line; unexpected unindent.
Documentation/x86/amd-memory-encryption.rst:128: WARNING: Block quote ends without a blank line; unexpected unindent.

You can repro by doing "make htmldocs".

-- 
Regards/Gruss,
    Boris.

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