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

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Message-ID: <ZkURaxF57kybgMm0@google.com>
Date: Wed, 15 May 2024 12:48:22 -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>, "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, May 15, 2024, Rick P Edgecombe wrote:
> On Wed, 2024-05-15 at 11:09 -0700, Sean Christopherson wrote:
> > 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.
> 
> Ok. The split seemed a little odd and special. I'm not sure it's the most
> interesting thing in the world, but there was some debate internally about it.
> 
> > 
> > If the host is accessing TDX private memory, we have far, far bigger problems
> > than aliased memtypes.
> 
> This wasn't the concern.
> 
> >   And so the fact that TDX private memory is forced WB is
> > interesting only to the guest, not KVM.
> 
> It's just another little quirk in an already complicated solution. They third
> thing we discussed was somehow rejecting or not supporting non-coherent DMA.
> This seemed simpler than that.

Again, huh?  This has _nothing_ to do with non-coherent DMA.  Devices can't DMA
into TDX private memory.