linux-kernel - Re: [PATCH v3 3/4] crypto: ccp - Add an API to return the supported SEV-SNP policy bits

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Message-ID: <aPpZrpfes8-SY4k_@google.com>
Date: Thu, 23 Oct 2025 09:37:02 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org, 
	linux-crypto@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Michael Roth <michael.roth@....com>, Ashish Kalra <ashish.kalra@....com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, David Miller <davem@...emloft.net>
Subject: Re: [PATCH v3 3/4] crypto: ccp - Add an API to return the supported
 SEV-SNP policy bits

On Wed, Oct 22, 2025, Tom Lendacky wrote:
> Supported policy bits are dependent on the level of SEV firmware that is
> currently running. Create an API to return the supported policy bits for
> a given level of firmware. KVM will AND that value with the KVM supported

Given "KVM will AND" and the shortlog, I expected a _future_ patch to have the 
           ^^^^
KVM changes.

That's partly a PEBKAC on my end (I mean, it's literally the first diff), but I
do think it highlights that we should probably separate the KVM change from the
PSP support. 

Hmm, actually, the patch ordering is bad.  There shouldn't need to be a separate
KVM change after this commit, because as things are ordered now, there will be an
ABI change between patch 1 and this patch.

So I think what you want is:

  1. KVM: SEV: Consolidate the SEV policy bits in a single header file
  2. crypto: ccp - Add an API to return the supported SEV-SNP policy bits
  3. KVM: SEV: Publish supported SEV-SNP policy bits
  4. KVM: SEV: Add known supported SEV-SNP policy bits

where #3 uses sev_get_snp_policy_bits() straightaway.

> policy bits to generate the actual supported policy bits.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/kvm/svm/sev.c       |  3 ++-
>  drivers/crypto/ccp/sev-dev.c | 37 ++++++++++++++++++++++++++++++++++++
>  include/linux/psp-sev.h      | 20 +++++++++++++++++++
>  3 files changed, 59 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 45e87d756e15..24167178bf05 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3099,7 +3099,8 @@ void __init sev_hardware_setup(void)
>  			sev_snp_supported = is_sev_snp_initialized();
>  
>  		if (sev_snp_supported) {
> -			snp_supported_policy_bits = KVM_SNP_POLICY_MASK_VALID;
> +			snp_supported_policy_bits = sev_get_snp_policy_bits();
> +			snp_supported_policy_bits &= KVM_SNP_POLICY_MASK_VALID;

I vote for:

			snp_supported_policy_bits = sev_get_snp_policy_bits() &
						    KVM_SNP_POLICY_MASK_VALID;

which makes it visually easier to see the policy bits logic.
	
>  			nr_ciphertext_hiding_asids = init_args.max_snp_asid;
>  		}