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

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Message-ID: <Z6YsWiTGM___898F@google.com>
Date: Fri, 7 Feb 2025 07:52:58 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: Vasant Hegde <vasant.hegde@....com>, pbonzini@...hat.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, thomas.lendacky@....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, nikunj@....com, ardb@...nel.org, 
	kevinloughlin@...gle.com, Neeraj.Upadhyay@....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 v3 3/3] x86/sev: Fix broken SNP support with KVM module built-in

On Wed, Feb 05, 2025, Ashish Kalra wrote:
> On 2/5/2025 1:31 PM, Sean Christopherson wrote:
> > On Wed, Feb 05, 2025, Vasant Hegde wrote:
> >> 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.
> > 
> 
> As i mentioned earlier and as part of this series and summarizing this again here:

Thanks!

> - snp_rmptable_init() enables SNP support system-wide and that means the HW starts
> doing RMP checks for memory accesses, but as RMP table is zeroed out initially, 
> all memory is configured to be host/HV owned. 
> 
> It is only after SNP_INIT(_EX) that RMP table is configured and initialized with
> HV_Fixed, firmware pages and stuff like IOMMU RMP enforcement is enabled. 
> 
> If the IOMMU initialization fails after IOMMU support on SNP check is completed
> and host SNP is enabled, then SNP_INIT(_EX) will fail as IOMMUs need to be enabled
> for SNP_INIT to succeed.
> 
> > 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.
> 
> Yes, that is true and this behavior is still consistent with this series.
> 
> Again to reiterate, if iommu_snp_enable() and host SNP enablement is successful,
> any late IOMMU initialization failures should cause SNP_INIT to fail and that means
> IOMMU RMP enforcement will never get enabled and RMP table will remain configured
> for all memory marked as HV/host owned. 

So the kernel should be able to limp along, but CC_ATTR_HOST_SEV_SNP will be in
a half-baked state.

Would it make sense to WARN if the RMP has been configured?  E.g. as a follow-up
change:

	/*
	 * SNP platform initilazation requires IOMMUs to be fully configured.
	 * If the RMP has NOT been configured, simply mark SNP as unsupported.
	 * If the RMP is configured, but RMP enforcement has not been enabled
	 * in IOMMUs, then the system is in a half-baked state, but can limp
	 * along as all memory should be Hypervisor-Owned in the RMP.   WARN,
	 * but leave SNP as "supported" to avoid confusing the kernel.
	 */
	if (ret && cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
	    !WARN_ON_ONCE(amd_iommu_snp_en))
		cc_platform_clear(CC_ATTR_HOST_SEV_SNP);