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

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Message-ID: <Z6OA9OhxBgsTY2ni@google.com>
Date: Wed, 5 Feb 2025 07:17:08 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Vasant Hegde <vasant.hegde@....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

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.

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.

> May be below check is enough?
> 
> 	if (ret && amd_iommu_snp_en)
> 
> 
> -Vasant
> 
>