Re: [PATCH v2 4/4] iommu/amd: Enable Host SNP support after enabling IOMMU SNP support

[Sean Christopherson <seanjc@google.com>]

On Fri, Jan 31, 2025, Ashish Kalra wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> This patch fixes the current SNP host enabling code and effectively SNP
  ^^^^^^^^^^
> ---
>  drivers/iommu/amd/init.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index c5cd92edada0..ee887aa4442f 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)

If you're feeling nitpicky, adding "__init" could be done in a separate patch.

>  {
>  #ifdef CONFIG_KVM_AMD_SEV
>  	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
> @@ -3219,6 +3219,11 @@ static void iommu_snp_enable(void)
>  		goto disable_snp;
>  	}
>  
> +	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;
>  
> @@ -3426,18 +3431,23 @@ void __init amd_iommu_detect(void)
>  	int ret;
>  
>  	if (no_iommu || (iommu_detected && !gart_iommu_aperture))
> -		return;
> +		goto disable_snp;
>  
>  	if (!amd_iommu_sme_check())
> -		return;
> +		goto disable_snp;
>  
>  	ret = iommu_go_to_state(IOMMU_IVRS_DETECTED);
>  	if (ret)
> -		return;
> +		goto disable_snp;

This handles initial failure, but it won't handle the case where amd_iommu_prepare()
fails, as the iommu_go_to_state() call from amd_iommu_enable() will get
short-circuited.  I don't see any pleasant options.  Maybe this?

diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index c5cd92edada0..436e47f13f8f 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -3318,6 +3318,8 @@ 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))
+               cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
        return ret;
 }
 

Somewhat of a side topic, what happens if the RMP is fully configured and _then_
IOMMU initialization fails?  I.e. if amd_iommu_init_pci() or amd_iommu_enable_interrupts()
fails?  Is that even survivable?

>  
>  	amd_iommu_detected = true;
>  	iommu_detected = 1;
>  	x86_init.iommu.iommu_init = amd_iommu_init;
> +	return;
> +
> +disable_snp:
> +	if (cc_platform_has(CC_ATTR_HOST_SEV_SNP))
> +		cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
>  }
>  
>  /****************************************************************************
> -- 
> 2.34.1
>