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 13:52:19 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "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>, "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, May 16, 2024 at 01:40:41PM +1200, Huang, Kai wrote:
> 
> 
> On 16/05/2024 1:20 pm, Edgecombe, Rick P wrote:
> > On Thu, 2024-05-16 at 13:04 +1200, Huang, Kai wrote:
> > > 
> > > I really don't see difference between ...
> > > 
> > >          is_private_mem(gpa)
> > > 
> > > ... and
> > > 
> > >          is_private_gpa(gpa)
> > > 
> > > If it confuses me, it can confuses other people.
> > 
> > Again, point taken. I'll try to think of a better name. Please share if you do.
> > 
> > > 
> > > The point is there's really no need to distinguish the two.  The GPA is
> > > only meaningful when it refers to the memory that it points to.
> > > 
> > > So far I am not convinced we need this helper, because such info we can
> > > already get from:
> > > 
> > >     1) fault->is_private;
> > >     2) Xarray which records memtype for given GFN.
> > > 
> > > So we should just get rid of it.
> > 
> > Kai, can you got look through the dev branch a bit more before making the same
> > point on every patch?
> > 
> > kvm_is_private_gpa() is used to set PFERR_PRIVATE_ACCESS, which in turn sets
> > fault->is_private. So you are saying we can use these other things that are
> > dependent on it. Look at the other callers too.
> 
> Well, I think I didn't make myself clear.
> 
> I don't object to have this helper.  If it helps, then we can have it.
> 
> My objection is the current implementation of it, because it is
> *conceptually* wrong for SEV-SNP.
> 
> Btw, I just look at the dev branch.
> 
> For the common code, it is used in kvm_tdp_mmu_map() and
> kvm_tdp_mmu_fast_pf_get_last_sptep() to get whether a GPA is private.
> 
> As said above, I don't see why we need a helper with the "current
> implementation" (which consults kvm_shared_gfn_mask()) for them.  We can
> just use fault->gfn + fault->is_private for such purpose.
What about a name like kvm_is_private_and_mirrored_gpa()?
Only TDX's private memory is mirrored and the common code needs a way to
tell that.


> It is also used in the TDX code like TDX variant handle_ept_violation() and
> tdx_vcpu_init_mem_region().  For them to be honest I don't quite care
> whether a helper is used.  We can have a helper if we have multiple callers,
> but this helper should be in TDX code, but not common MMU code.
>