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

[Dave Hansen]

On 3/25/21 8:31 AM, Brijesh Singh wrote:
> 
> 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.

I think you're confusing the CPUID bit that initially populates
X86_FEATURE_SEV_SNP with the X86_FEATURE bit.  We clear X86_FEATURE bits
all the time for features that the kernel turns off, even while the
hardware supports it.

Look at what we do in init_ia32_feat_ctl() for SGX, for instance.  We
then go on to use X86_FEATURE_SGX at runtime to see if SGX was disabled,
even though the hardware supports it.

>> 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.

IS_ENABLED() tends to be nicer for these things.

Even better is if you coordinate these with your X86_FEATURE_SEV_SNP
checks.  Then, put X86_FEATURE_SEV_SNP in disabled-features.h, and you
can use cpu_feature_enabled(X86_FEATURE_SEV_SNP) as both a
(statically-patched) runtime *AND* compile-time check without an
explicit #ifdefs.