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

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Message-ID: <d438eb4b-c3b1-4318-8db3-b84443482e1c@amd.com>
Date: Fri, 7 Feb 2025 15:45:54 -0600
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Sean Christopherson <seanjc@...gle.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

Hello Sean,

On 2/7/2025 9:52 AM, Sean Christopherson wrote:
> 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);

Yes, i can re-spin the series with this WARN_ON() added and additional comments added.

Thanks,
Ashish