Re: [PATCH 1/4] iommu/amd: Check SNP support before enabling IOMMU

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 27, 2025, Ashish Kalra wrote:
> Hello Sean,
> 
> On 1/24/2025 6:39 PM, Sean Christopherson wrote:
> > On Fri, Jan 24, 2025, Ashish Kalra wrote:
> >> With discussions with the AMD IOMMU team, here is the AMD IOMMU
> >> initialization flow:
> > 
> > ..
> > 
> >> IOMMU SNP check
> >>   Core IOMMU subsystem init is done during iommu_subsys_init() via
> >>   subsys_initcall.  This function does change the DMA mode depending on
> >>   kernel config.  Hence, SNP check should be done after subsys_initcall.
> >>   That's why its done currently during IOMMU PCI init (IOMMU_PCI_INIT stage).
> >>   And for that reason snp_rmptable_init() is currently invoked via
> >>   device_initcall().
> >>  
> >> The summary is that we cannot move snp_rmptable_init() to subsys_initcall as
> >> core IOMMU subsystem gets initialized via subsys_initcall.
> > 
> > Just explicitly invoke RMP initialization during IOMMU SNP setup.  Pretending
> > there's no connection when snp_rmptable_init() checks amd_iommu_snp_en and has
> > a comment saying it needs to come after IOMMU SNP setup is ridiculous.
> > 
> 
> Thanks for the suggestion and the patch, i have tested it works for all cases
> and scenarios. I will post the next version of the patch-set based on this
> patch.

One thing I didn't account for: if IOMMU initialization fails and iommu_snp_enable()
is never reached, CC_ATTR_HOST_SEV_SNP will be left set.

I don't see any great options.  Something like the below might work?  And maybe
keep a device_initcall() in arch/x86/virt/svm/sev.c that sanity checks that SNP
really is fully enabled?  Dunno, hopefully someone has a better idea.

diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 0e0a531042ac..6d62ee8e0055 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -3295,6 +3295,9 @@ static int __init iommu_go_to_state(enum iommu_init_state state)
                ret = state_next();
        }
 
+       if (ret && !amd_iommu_snp_en)
+               cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
+
        return ret;
 }