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

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 05, 2025, Vasant Hegde wrote:
> On 2/5/2025 8:47 PM, Sean Christopherson wrote:
> > On Wed, Feb 05, 2025, Vasant Hegde wrote:
> >>> @@ -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.
> > 
> > That doesn't address the case where amd_iommu_prepare() fails, because amd_iommu_snp_en
> > will be %false (its init value) and the RMP will be uninitialized, i.e.
> > CC_ATTR_HOST_SEV_SNP will be incorrectly left set.
> 
> You are right. I missed early failure scenarios :-(
> 
> > 
> > And conversely, IMO clearing CC_ATTR_HOST_SEV_SNP after initializing the IOMMU
> > and RMP is wrong as well.  Such a host is probably hosed regardless, but from
> > the CPU's perspective, SNP is supported and enabled.
> 
> So we don't want to clear  CC_ATTR_HOST_SEV_SNP after RMP initialization -OR-
> clear for all failures?

I honestly don't know, because the answer largely depends on what happens with
hardware.  I asked in an earlier version of this series if IOMMU initialization
failure after the RMP is configured is even survivable.

For this series, I think it makes sense to match the existing behavior, unless
someone from AMD can definitively state that we should do something different.
And the existing behavior is that amd_iommu_snp_en and CC_ATTR_HOST_SEV_SNP will
be left set if the IOMMU completes iommu_snp_enable(), and the kernel completes
RMP setup.