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 12:41:21 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org, linux-crypto@...r.kernel.org
Cc:     brijesh.singh@....com, 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


On 3/25/21 10:51 AM, Dave Hansen wrote:
> 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.


Ah, yes I was getting mixed up. I will see if we can remove the
snp_key_enabled and use the feature check.


> 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.

I will try improve this in v2 and will try IS_ENABLED().