Re: [PATCH v3 3/3] x86/sev: Fix broken SNP support with KVM module built-in

[Vasant Hegde]

Hi Ashish,

[Sorry. I didn't see this series and responded to v2].

On 2/4/2025 3:26 AM, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> 
> Fix issues with enabling SNP host support and effectively SNP support
> which is broken with respect to the KVM module being built-in.
> 
> SNP host support is enabled in snp_rmptable_init() which is invoked as
> device_initcall(). SNP check on IOMMU is done during IOMMU PCI init
> (IOMMU_PCI_INIT stage). And for that reason snp_rmptable_init() is
> currently invoked via device_initcall() and cannot be invoked via
> subsys_initcall() as core IOMMU subsystem gets initialized via
> subsys_initcall().
> 
> Now, if kvm_amd module is built-in, it gets initialized before SNP host
> support is enabled in snp_rmptable_init() :
> 
> [   10.131811] kvm_amd: TSC scaling supported
> [   10.136384] kvm_amd: Nested Virtualization enabled
> [   10.141734] kvm_amd: Nested Paging enabled
> [   10.146304] kvm_amd: LBR virtualization supported
> [   10.151557] kvm_amd: SEV enabled (ASIDs 100 - 509)
> [   10.156905] kvm_amd: SEV-ES enabled (ASIDs 1 - 99)
> [   10.162256] kvm_amd: SEV-SNP enabled (ASIDs 1 - 99)
> [   10.171508] kvm_amd: Virtual VMLOAD VMSAVE supported
> [   10.177052] kvm_amd: Virtual GIF supported
> ...
> ...

.../...

> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index c5cd92edada0..4bcb474e2252 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -3194,7 +3194,7 @@ static bool __init detect_ivrs(void)
>  	return true;
>  }
>  
> -static void iommu_snp_enable(void)
> +static __init void iommu_snp_enable(void)
>  {
>  #ifdef CONFIG_KVM_AMD_SEV
>  	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
> @@ -3219,6 +3219,14 @@ static void iommu_snp_enable(void)
>  		goto disable_snp;
>  	}
>  
> +	/*
> +	 * Enable host SNP support once SNP support is checked on IOMMU.
> +	 */
> +	if (snp_rmptable_init()) {
> +		pr_warn("SNP: RMP initialization failed, SNP cannot be supported.\n");
> +		goto disable_snp;
> +	}
> +
>  	pr_info("IOMMU SNP support enabled.\n");
>  	return;
>  
> @@ -3318,6 +3326,9 @@ static int __init iommu_go_to_state(enum iommu_init_state state)
>  		ret = state_next();
>  	}
>  
> +	if (ret && !amd_iommu_snp_en && cc_platform_has(CC_ATTR_HOST_SEV_SNP))


I think we should clear when `amd_iommu_snp_en` is true. May be below check is
enough?

	if (ret && amd_iommu_snp_en)


-Vasant