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

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Message-ID: <9bc661643e3ce11f32f0bac78a2dbfd62d9cd283.camel@intel.com>
Date: Wed, 22 May 2024 23:09:54 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "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, 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.