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

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Message-ID: <8f7822df-466d-497c-9c41-77524b2870b6@amd.com>
Date: Wed, 5 Feb 2025 22:25:18 +0530
From: Vasant Hegde <vasant.hegde@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Ashish Kalra <Ashish.Kalra@....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

Hi Sean,


On 2/5/2025 8:47 PM, Sean Christopherson wrote:
> On Wed, Feb 05, 2025, Vasant Hegde wrote:
>> Hi Ashish,
>>
>> [Sorry. I didn't see this series and responded to v2].
> 
> Heh, and then I saw your other email first and did the same.  Copying my response
> here, too (and fixing a few typos in the process).
> 
>>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>>> index c5cd92edada0..4bcb474e2252 100644
>>> --- a/drivers/iommu/amd/init.c
>>> +++ b/drivers/iommu/amd/init.c
>>> @@ -3194,7 +3194,7 @@ static bool __init detect_ivrs(void)
>>>  	return true;
>>>  }
>>>  
>>> -static void iommu_snp_enable(void)
>>> +static __init void iommu_snp_enable(void)
>>>  {
>>>  #ifdef CONFIG_KVM_AMD_SEV
>>>  	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
>>> @@ -3219,6 +3219,14 @@ static void iommu_snp_enable(void)
>>>  		goto disable_snp;
>>>  	}
>>>  
>>> +	/*
>>> +	 * Enable host SNP support once SNP support is checked on IOMMU.
>>> +	 */
>>> +	if (snp_rmptable_init()) {
>>> +		pr_warn("SNP: RMP initialization failed, SNP cannot be supported.\n");
>>> +		goto disable_snp;
>>> +	}
>>> +
>>>  	pr_info("IOMMU SNP support enabled.\n");
>>>  	return;
>>>  
>>> @@ -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?

-Vasant