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

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Message-ID: <e27a4198-ee94-4ca1-9973-1f6164ed4e64@amd.com>
Date: Tue, 10 Dec 2024 19:01:49 -0600
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Sean Christopherson <seanjc@...gle.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 12/10/2024 6:48 PM, Kalra, Ashish wrote:
> 
> 
> On 12/10/2024 4:57 PM, Sean Christopherson wrote:
>> 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()?
> 
> One reason is the need to do SEV SHUTDOWN before SNP_SHUTDOWN if any SEV VMs are active
> and this is taken care with the single API interface sev_platform_shutdown(), so that's 
> why considering using a consistent API interface for both INIT and SHUTDOWN ...
> - sev_platform_init()
> - sev_platform_shutdown()

Which also assists in using the same internal interface __sev_firmware_shutdown()
to be called both with sev_platform_shutdown() and the SNP panic notifier to shutdown
both SEV and SNP (in that order). 

Thanks,
Ashish

> 
> We can use separate APIs, but then we probably need the same for shutdown too and KVM
> will need to keep track of any active SEV VMs and ensure to call sev_platform_shutdown()
> before sev_snp_platform_shutdown() (as part of sev_hardware_unsetup()).
> 
> Thanks,
> Ashish
> 
>>
>> 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))
>