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

[Brijesh Singh]

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.


>