linux-kernel - Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn

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Date: Wed, 15 May 2024 15:56:19 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "Aktas, Erdem" <erdemaktas@...gle.com>, "Zhao,
 Yan Y" <yan.y.zhao@...el.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>
Subject: Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn_range() is
 called for TDX

On Wed, 2024-05-15 at 08:49 -0700, Rick Edgecombe wrote:
> On Wed, 2024-05-15 at 08:34 -0700, Sean Christopherson wrote:
> > On Tue, May 14, 2024, Rick Edgecombe wrote:
> > > When virtualizing some CPU features, KVM uses kvm_zap_gfn_range() to zap
> > > guest mappings so they can be faulted in with different PTE properties.
> > > 
> > > For TDX private memory this technique is fundamentally not possible.
> > > Remapping private memory requires the guest to "accept" it, and also the
> > > needed PTE properties are not currently supported by TDX for private
> > > memory.
> > > 
> > > These CPU features are:
> > > 1) MTRR update
> > > 2) CR0.CD update
> > > 3) Non-coherent DMA status update
> > 
> > Please go review the series that removes these disaster[*], I suspect it
> > would
> > literally have taken less time than writing this changelog :-)
> > 
> > [*] https://lore.kernel.org/all/20240309010929.1403984-1-seanjc@google.com
> 
> We have one additional detail for TDX in that KVM will have different cache
> attributes between private and shared. Although implementation is in a later
> patch, that detail has an affect on whether we need to support zapping in the
> basic MMU support.

Or most specifically, we only need this zapping if we *try* to have consistent
cache attributes between private and shared. In the non-coherent DMA case we
can't have them be consistent because TDX doesn't support changing the private
memory in this way.