linux-kernel - Re: [PATCH v2 3/3] x86/sev: Add SEV-SNP CipherTextHiding support

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Message-ID: <Z1jHZDevvjWFQo5A@google.com>
Date: Tue, 10 Dec 2024 14:57:40 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: Tom Lendacky <thomas.lendacky@....com>, Peter Gonda <pgonda@...gle.com>, pbonzini@...hat.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, hpa@...or.com, herbert@...dor.apana.org.au, 
	x86@...nel.org, john.allen@....com, davem@...emloft.net, michael.roth@....com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2 3/3] x86/sev: Add SEV-SNP CipherTextHiding support

On Tue, Dec 10, 2024, Ashish Kalra wrote:
> On 12/9/2024 7:30 PM, Sean Christopherson wrote:
> > Why can't we simply separate SNP initialization from SEV+ initialization?
> 
> Yes we can do that, by default KVM module load time will only do SNP initialization,
> and then we will do SEV initialization if a SEV VM is being launched.
> 
> This will remove the probe parameter from init_args above, but will need to add another
> parameter like VM type to specify if SNP or SEV initialization is to be performed with
> the sev_platform_init() call.

Any reason not to simply use separate APIs?  E.g. sev_snp_platform_init() and
sev_platform_init()?

And if the cc_platform_has(CC_ATTR_HOST_SEV_SNP) check is moved inside of
sev_snp_platform_init() (probably needs to be there anyways), then the KVM code
is quite simple and will undergo minimal churn.

E.g.

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 5e4581ed0ef1..7e75bc55d017 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -404,7 +404,6 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
                            unsigned long vm_type)
 {
        struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
-       struct sev_platform_init_args init_args = {0};
        bool es_active = vm_type != KVM_X86_SEV_VM;
        u64 valid_vmsa_features = es_active ? sev_supported_vmsa_features : 0;
        int ret;
@@ -444,8 +443,7 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
        if (ret)
                goto e_no_asid;
 
-       init_args.probe = false;
-       ret = sev_platform_init(&init_args);
+       ret = sev_platform_init();
        if (ret)
                goto e_free;
 
@@ -3053,7 +3051,7 @@ void __init sev_hardware_setup(void)
        sev_es_asid_count = min_sev_asid - 1;
        WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV_ES, sev_es_asid_count));
        sev_es_supported = true;
-       sev_snp_supported = sev_snp_enabled && cc_platform_has(CC_ATTR_HOST_SEV_SNP);
+       sev_snp_supported = sev_snp_enabled && !sev_snp_platform_init();
 
 out:
        if (boot_cpu_has(X86_FEATURE_SEV))