linux-kernel - Re: [PATCH] KVM: SEV: Disable SEV-SNP support on initialization failure

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Message-ID: <30ec902e-821f-7b26-4316-ca0e51ec4a03@amd.com>
Date: Fri, 9 May 2025 09:04:42 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ashish Kalra <Ashish.Kalra@....com>, seanjc@...gle.com,
 pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, shuah@...nel.org, prsampat@....com
Cc: pgonda@...gle.com, nikunj@....com, pankaj.gupta@....com,
 michael.roth@....com, sraithal@....com, linux-kernel@...r.kernel.org,
 x86@...nel.org, kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-coco@...ts.linux.dev
Subject: Re: [PATCH] KVM: SEV: Disable SEV-SNP support on initialization
 failure

On 5/8/25 17:52, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> During platform init, SNP initialization may fail for several reasons,
> such as firmware command failures and incompatible versions. However,
> the KVM capability may continue to advertise support for it.
> 
> The platform may have SNP enabled but if SNP_INIT fails then SNP is
> not supported by KVM.
> 
> During KVM module initialization query the SNP platform status to obtain
> the SNP initialization state and use it as an additional condition to
> determine support for SEV-SNP.
> 
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Co-developed-by: Pratik R. Sampat <prsampat@....com>
> Signed-off-by: Pratik R. Sampat <prsampat@....com>
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> ---
>  arch/x86/kvm/svm/sev.c | 43 +++++++++++++++++++++++++++++++++---------
>  1 file changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index ada53f04158c..a6abdb26f877 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2934,6 +2934,32 @@ void __init sev_set_cpu_caps(void)
>  	}
>  }
>  
> +static bool sev_is_snp_initialized(void)
> +{
> +	struct sev_user_data_snp_status *status;
> +	struct sev_data_snp_addr buf;
> +	bool initialized = false;
> +	void *data;

No need for 'data', just allocate directly to 'status', no?

> +	int error;
> +

	if (!sev_snp_supported)
		return false;

No need to issue the command if it doesn't matter.

> +	data = snp_alloc_firmware_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);

GFP_KERNEL instead of GFP_KERNEL_ACCOUNT ?

> +	if (!data)
> +		return initialized;

		return false;

I like explicit values in these conditions, but that's just me.

> +
> +	buf.address = __psp_pa(data);
> +	if (sev_do_cmd(SEV_CMD_SNP_PLATFORM_STATUS, &buf, &error))

You should issue an error message here or not pass in error (I would
prefer the former).

> +		goto out;
> +
> +	status = (struct sev_user_data_snp_status *)data;
> +	if (status->state)
> +		initialized = true;

	initialized = !!status->state; ?

> +
> +out:
> +	snp_free_firmware_page(data);
> +
> +	return initialized;
> +}
> +
>  void __init sev_hardware_setup(void)
>  {
>  	unsigned int eax, ebx, ecx, edx, sev_asid_count, sev_es_asid_count;
> @@ -3038,6 +3064,14 @@ void __init sev_hardware_setup(void)
>  	sev_snp_supported = sev_snp_enabled && cc_platform_has(CC_ATTR_HOST_SEV_SNP);
>  
>  out:
> +	if (sev_enabled) {
> +		init_args.probe = true;
> +		if (sev_platform_init(&init_args))
> +			sev_supported = sev_es_supported = sev_snp_supported = false;
> +		else
> +			sev_snp_supported &= sev_is_snp_initialized();

With changes above, then you can just do:

	sev_snp_supported = sev_is_snp_initialized()

Thanks,
Tom

> +	}
> +
>  	if (boot_cpu_has(X86_FEATURE_SEV))
>  		pr_info("SEV %s (ASIDs %u - %u)\n",
>  			sev_supported ? min_sev_asid <= max_sev_asid ? "enabled" :
> @@ -3064,15 +3098,6 @@ void __init sev_hardware_setup(void)
>  	sev_supported_vmsa_features = 0;
>  	if (sev_es_debug_swap_enabled)
>  		sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
> -
> -	if (!sev_enabled)
> -		return;
> -
> -	/*
> -	 * Do both SNP and SEV initialization at KVM module load.
> -	 */
> -	init_args.probe = true;
> -	sev_platform_init(&init_args);
>  }
>  
>  void sev_hardware_unsetup(void)