linux-kernel - Re: [PATCH v2 4/4] iommu/amd: Enable Host SNP support after enabling IOMMU SNP support

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Message-ID: <9b338481-39c6-4102-9c4c-19793477242e@amd.com>
Date: Fri, 31 Jan 2025 16:53:07 -0600
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: 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 v2 4/4] iommu/amd: Enable Host SNP support after enabling
 IOMMU SNP support

Hello Sean,

On 1/30/2025 7:48 PM, Sean Christopherson wrote:
> On Fri, Jan 31, 2025, Ashish Kalra wrote:
>> From: Sean Christopherson <seanjc@...gle.com>
>>
>> This patch fixes the current SNP host enabling code and effectively SNP
>   ^^^^^^^^^^
>> ---
>>  drivers/iommu/amd/init.c | 18 ++++++++++++++----
>>  1 file changed, 14 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>> index c5cd92edada0..ee887aa4442f 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)
> 
> If you're feeling nitpicky, adding "__init" could be done in a separate patch.
> 
>>  {
>>  #ifdef CONFIG_KVM_AMD_SEV
>>  	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
>> @@ -3219,6 +3219,11 @@ static void iommu_snp_enable(void)
>>  		goto disable_snp;
>>  	}
>>  
>> +	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;
>>  
>> @@ -3426,18 +3431,23 @@ void __init amd_iommu_detect(void)
>>  	int ret;
>>  
>>  	if (no_iommu || (iommu_detected && !gart_iommu_aperture))
>> -		return;
>> +		goto disable_snp;
>>  
>>  	if (!amd_iommu_sme_check())
>> -		return;
>> +		goto disable_snp;
>>  
>>  	ret = iommu_go_to_state(IOMMU_IVRS_DETECTED);
>>  	if (ret)
>> -		return;
>> +		goto disable_snp;
> 
> This handles initial failure, but it won't handle the case where amd_iommu_prepare()
> fails, as the iommu_go_to_state() call from amd_iommu_enable() will get
> short-circuited.  I don't see any pleasant options.  Maybe this?
> 
> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
> index c5cd92edada0..436e47f13f8f 100644
> --- a/drivers/iommu/amd/init.c
> +++ b/drivers/iommu/amd/init.c
> @@ -3318,6 +3318,8 @@ 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))
> +               cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
>         return ret;
>  }
>  

This is surely hackish but it will work.

Though to add here, as mentioned in the patch commit logs, IOMMU needs to enabled for SNP_INIT to succeed. 

If IOMMU initialization fails and iommu_snp_enable() is never reached, CC_ATTR_HOST_SEV_SNP will be
left set but that will cause PSP driver's SNP_INIT to fail with invalid configuration error as 
IOMMU SNP sanity checks in SNP firmware will fail (if IOMMUs are not enabled) as below: 

[    9.723114] ccp 0000:23:00.1: sev enabled
[    9.727602] ccp 0000:23:00.1: psp enabled
[    9.732527] ccp 0000:a2:00.1: enabling device (0000 -> 0002)
[    9.739098] ccp 0000:a2:00.1: no command queues available
[    9.745167] ccp 0000:a2:00.1: psp enabled
[    9.805337] ccp 0000:23:00.1: SEV-SNP: failed to INIT rc -5, error 0x3
[    9.866426] ccp 0000:23:00.1: SEV API:1.53 build:5
...
and that will cause CC_ATTR_HOST_SEV_SNP flag to be cleared. 

> 
> Somewhat of a side topic, what happens if the RMP is fully configured and _then_
> IOMMU initialization fails?  I.e. if amd_iommu_init_pci() or amd_iommu_enable_interrupts()
> fails?  Is that even survivable?

Actually, SNP firmware initializes the IOMMU to perform RMP enforcement during SNP_INIT,
but as i mentioned above if IOMMU is not enabled then SNP_INIT will fail. 

Thanks,
Ashish

> 
>>  
>>  	amd_iommu_detected = true;
>>  	iommu_detected = 1;
>>  	x86_init.iommu.iommu_init = amd_iommu_init;
>> +	return;
>> +
>> +disable_snp:
>> +	if (cc_platform_has(CC_ATTR_HOST_SEV_SNP))
>> +		cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
>>  }
>>  
>>  /****************************************************************************
>> -- 
>> 2.34.1
>>