linux-kernel - Re: [PATCH v2 3/4] x86/cpu: clear SME features when not in use

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Message-ID: <56dbbd52-93bd-2c70-3862-3b62231a4024@amd.com>
Date:   Fri, 11 Feb 2022 15:17:16 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Mario Limonciello <mario.limonciello@....com>,
        Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, hughsient@...il.com,
        Martin Fernandez <martin.fernandez@...ypsium.com>,
        linux-kernel@...r.kernel.org, Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v2 3/4] x86/cpu: clear SME features when not in use

On 2/11/22 15:02, Mario Limonciello wrote:
> Currently the SME CPU feature flag is reflective of whether the CPU
> supports the features but not whether they have been activated by the
> kernel.
> 
> Change this around to clear the features if the kernel is not using
> them so userspace can determine if they are available and in use
> from `/proc/cpuinfo`.
> 
> As the feature flag is now cleared on systems that SME isn't active
> use CPUID 0x8000001f to confirm SME availability before calling
> `native_wbinvd`.
> 
> Signed-off-by: Mario Limonciello <mario.limonciello@....com>
> ---

>   
> +		if (!sme_me_mask)
> +			goto clear_all;
> +

This is incorrect. Just because host memory encryption is not active 
doesn't mean that SEV guests can't be run. This should only clear the SME 
feature bit.

Thanks,
Tom