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    SubjectRe: [RFC Part2 PATCH 01/30] x86: Add the host SEV-SNP initialization support
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    On 3/25/21 9:58 AM, Dave Hansen wrote:
    >> +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?


    The X86_FEATURE_SEV_SNP indicates that hardware supports the feature --
    this does not necessary means that SEV-SNP is enabled in the host. The
    snp_enabled_key() helper is later used by kernel and drivers to check
    whether SEV-SNP is enabled. e.g. when a driver calls the RMPUPDATE
    instruction, the rmpupdate helper routine checks whether the SNP is
    enabled. If SEV-SNP is not enabled then instruction will cause a #UD.

    >
    > 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.


    Noted, I will add the #ifdef  to make sure that its compiled out when
    the config does not have the AMD_MEM_ENCRYPTION enabled.


    >

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