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

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Message-ID: <ZkT4RC_l_F_9Rk-M@google.com>
Date: Wed, 15 May 2024 11:09:09 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.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>, Erdem Aktas <erdemaktas@...gle.com>, 
	Yan Zhao <yan.y.zhao@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn_range() is
 called for TDX

On Wed, May 15, 2024, Rick P Edgecombe wrote:
> On Wed, 2024-05-15 at 09:02 -0700, Sean Christopherson wrote:
> > > 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.
> > 
> > Huh?  That makes no sense.  A physical page can't be simultaneously mapped
> > SHARED and PRIVATE, so there can't be meaningful cache attribute aliasing
> > between private and shared EPT entries.
> > 
> > Trying to provide consistency for the GPA is like worrying about having
> > matching PAT entires for the virtual address in two different processes.
> 
> No, not matching between the private and shared mappings of the same page The
> whole private memory will be WB, and the whole shared half will honor PAT.

I'm still failing to see why that's at all interesting.  The non-coherent DMA
trainwreck is all about KVM worrying about the guest and host having different
memtypes for the same physical page.

If the host is accessing TDX private memory, we have far, far bigger problems
than aliased memtypes.  And so the fact that TDX private memory is forced WB is
interesting only to the guest, not KVM.