linux-kernel - Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for TDX shared bit of GPA

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Date: Thu, 16 May 2024 00:35:35 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>
CC: "sagis@...gle.com" <sagis@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for
 TDX shared bit of GPA

On Thu, 2024-05-16 at 12:25 +1200, Huang, Kai wrote:
> 
> 
> On 16/05/2024 12:19 pm, Edgecombe, Rick P wrote:
> > On Thu, 2024-05-16 at 12:12 +1200, Huang, Kai wrote:
> > > 
> > > I don't have strong objection if the use of kvm_gfn_shared_mask() is
> > > contained in smaller areas that truly need it.  Let's discuss in
> > > relevant patch(es).
> > > 
> > > However I do think the helpers like below makes no sense (for SEV-SNP):
> > > 
> > > +static inline bool kvm_is_private_gpa(const struct kvm *kvm, gpa_t gpa)
> > > +{
> > > +       gfn_t mask = kvm_gfn_shared_mask(kvm);
> > > +
> > > +       return mask && !(gpa_to_gfn(gpa) & mask);
> > > +}
> > 
> > You mean the name? SNP doesn't have a concept of "private GPA" IIUC. The C
> > bit
> > is more like an permission bit. So SNP doesn't have private GPAs, and the
> > function would always return false for SNP. So I'm not sure it's too
> > horrible.
> 
> Hmm.. Why SNP doesn't have private GPAs?  They are crypto-protected and 
> KVM cannot access directly correct?

I suppose a GPA could be pointing to memory that is private. But I think in SNP
it is more the memory that is private. Now I see more how it could be confusing.

> 
> > 
> > If it's the name, can you suggest something?
> 
> The name make sense, but it has to reflect the fact that a given GPA is 
> truly private (crypto-protected, inaccessible to KVM).

If this was a function that tested whether memory is private and took a GPA, I
would call it is_private_mem() or something. Because it's testing the memory and
takes a GPA, not testing the GPA. Usually a function name should be about what
the function does, not what arguments it takes.

I can't think of a better name, but point taken that it is not ideal. I'll try
to think of something.