linux-kernel - Re: [PATCH v11 18/35] KVM: SEV: Add KVM_SEV_SNP_LAUNCH

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Date: Fri, 9 Feb 2024 06:34:23 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org, 
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	vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org, 
	dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com, 
	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com, 
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	sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com, 
	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com, 
	pankaj.gupta@....com, liam.merwick@...cle.com, zhi.a.wang@...el.com, 
	Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v11 18/35] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command

On Thu, Feb 08, 2024, Michael Roth wrote:
> On Wed, Feb 07, 2024 at 12:43:02AM +0100, Paolo Bonzini wrote:
> > On Fri, Feb 2, 2024 at 11:55 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > What sanity is being checked for, in other words why are they useful?
> > If all you get for breaking the promise is a KVM_BUG_ON, for example,
> > that's par for the course. If instead you get an oops, then we have a
> > problem.
> > 
> > I may be a bit less draconian than Sean, but the assumptions need to
> > be documented and explained because they _are_ going to go away.
> 
> Maybe in this case sanity-check isn't the right word, but for instance
> the occurance Sean objected to:
> 
>   kvaddr = pfn_to_kaddr(pfns[i]);
>   if (!virt_addr_valid(kvaddr)) {
>     ...
>     ret = -EINVAL;
> 
> where there are pfn_valid() checks underneath the covers that provide
> some assurance this is normal struct-page-backed/kernel-tracked memory
> that has a mapping in the directmap we can use here. Dropping that
> assumption means we need to create temporary mappings to access the PFN,

No, you don't.  kvm_vcpu_map() does all of the lifting for you, with the small
caveat that it obviously needs a vCPU.  But that's trivial to solve with a minor
refactoring, *if* we need to solve that problem (it's not clear to me whether or
not the APIs for copying data into guest_memfd will be VM-scoped or vCPU-scoped).