Re: [PATCH v10 06/50] x86/sev: Add the host SEV-SNP initialization support

[Kalra, Ashish]

On 11/7/2023 4:08 PM, Borislav Petkov wrote:
>   static int __init snp_rmptable_init(void)
>   {
> -	int family, model;
> -
> -	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> +	if (!amd_iommu_snp_en)
>   		return 0;

We will still need some method to tell the IOMMU driver if SNP 
support/feature is disabled by this function, for example, when CPU 
family and model is not supported by SNP and we jump to no_snp label.

The reliable way for this to work is to ensure snp_rmptable_init() is 
called before IOMMU initialization and then IOMMU initialization depends 
on SNP feature flag setup by snp_rmptable_init() to enable SNP support 
on IOMMU or not.

If snp_rmptable_init() is called after IOMMU initialization and it 
detects an issue with SNP support it will clear the SNP feature but the 
IOMMU driver does not get notified about it, therefore, 
snp_rmptable_init() should get called before IOMMU initialization or as 
part of IOMMU initialization, for example, amd_iommu_enable() calling 
snp_rmptable_init() before calling iommu_snp_enable().

Thanks,
Ashish