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Date: Wed, 15 May 2024 16:26:16 -0700
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Subject: Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn_range() is
 called for TDX
From: Sean Christopherson <seanjc@google.com>
To: Rick P Edgecombe <rick.p.edgecombe@intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "sagis@google.com" <sagis@google.com>, 
	"isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>, Erdem Aktas <erdemaktas@google.com>, 
	Yan Zhao <yan.y.zhao@intel.com>, "pbonzini@redhat.com" <pbonzini@redhat.com>, 
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>, "dmatlack@google.com" <dmatlack@google.com>
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On Wed, May 15, 2024, Rick P Edgecombe wrote:
> On Wed, 2024-05-15 at 12:48 -0700, Sean Christopherson wrote:
> > > It's just another little quirk in an already complicated solution. Th=
ey
> > > third
> > > thing we discussed was somehow rejecting or not supporting non-cohere=
nt DMA.
> > > This seemed simpler than that.
> >=20
> > Again, huh?=C2=A0 This has _nothing_ to do with non-coherent DMA.=C2=A0=
 Devices can't
> > DMA
> > into TDX private memory.
>=20
> Hmm... I'm confused how you are confused... :)
>=20
> For normal VMs (after that change you linked), guests will honor guest PA=
T on
> newer HW. On older HW it will only honor guest PAT if non-coherent DMA is
> attached.
>=20
> For TDX we can't honor guest PAT for private memory. So we can either hav=
e:
> 1. Have shared honor PAT and private not.
> 2. Have private and shared both not honor PAT and be consistent. Unless n=
on-
> coherent DMA is attached. In that case KVM could zap shared only and swit=
ch to
> 1.

Oh good gravy, hell no :-)