linux-kernel - Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

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Date: Wed, 22 May 2024 16:47:54 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Wed, May 22, 2024 at 11:09:54PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> On Wed, 2024-05-22 at 15:34 -0700, Isaku Yamahata wrote:
> > option 1. Allow per-VM kvm_mmu_max_gfn()
> > Pro: Conceptually easy to understand and it's straightforward to disallow
> >      memslot creation > virtual maxphyaddr
> > Con: overkill for the corner case? The diff is attached.  This is only when
> > user
> >      space creates memlost > virtual maxphyaddr and the guest accesses GPA >
> >      virtual maxphyaddr)
> 
> It breaks the promise that gfn's don't have the share bit which is the pro for
> hiding the shared bit in the tdp mmu iterator.
> 
> > 
> > option 2. Keep kvm_mmu_max_gfn() and add ad hock address check.
> > Pro: Minimal change?
> >      Modify kvm_handel_noslot_fault() or kvm_faultin_pfn() to reject GPA >
> >      virtual maxphyaddr.
> > Con: Conceptually confusing with allowing operation on GFN > virtual
> > maxphyaddr.
> >      The change might be unnatural or ad-hoc because it allow to create
> > memslot
> >      with GPA > virtual maxphyaddr.
> 
> I can't find any actual functional problem to just ignoring it. Just some extra
> work to go over ranges that aren't covered by the root.
> 
> How about we leave option 1 as a separate patch and note it is not functionally
> required? Then we can shed it if needed. At the least it can serve as a
> conversation piece in the meantime.

Ok. We understand the situation correctly. I think it's okay to do nothing for
now with some notes somewhere as record because it doesn't affect much for usual
case.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>