linux-kernel - Re: [RFC Part2 PATCH 01/30] x86: Add the host SEV-SNP initialization support

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Date:   Thu, 25 Mar 2021 07:58:29 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        linux-crypto@...r.kernel.org
Cc:     ak@...ux.intel.com, herbert@...dor.apana.org.au,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC Part2 PATCH 01/30] x86: Add the host SEV-SNP initialization
 support

> +static int __init mem_encrypt_snp_init(void)
> +{
> +	if (!boot_cpu_has(X86_FEATURE_SEV_SNP))
> +		return 1;
> +
> +	if (rmptable_init()) {
> +		setup_clear_cpu_cap(X86_FEATURE_SEV_SNP);
> +		return 1;
> +	}
> +
> +	static_branch_enable(&snp_enable_key);
> +
> +	return 0;
> +}

Could you explain a bit why 'snp_enable_key' is needed in addition to
X86_FEATURE_SEV_SNP?

For a lot of features, we just use cpu_feature_enabled(), which does
both compile-time and static_cpu_has().  This whole series seems to lack
compile-time disables for the code that it adds, like the code it adds
to arch/x86/mm/fault.c or even mm/memory.c.