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Message-ID: <Z5f2rnMyBAjK88dP@google.com>
Date: Mon, 27 Jan 2025 13:12:14 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: Vasant Hegde <vasant.hegde@....com>, Tom Lendacky <thomas.lendacky@....com>, pbonzini@...hat.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	john.allen@....com, herbert@...dor.apana.org.au, davem@...emloft.net, 
	joro@...tes.org, suravee.suthikulpanit@....com, will@...nel.org, 
	robin.murphy@....com, michael.roth@....com, dionnaglaze@...gle.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-crypto@...r.kernel.org, linux-coco@...ts.linux.dev, 
	iommu@...ts.linux.dev
Subject: Re: [PATCH 1/4] iommu/amd: Check SNP support before enabling IOMMU

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;
 }

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