linux-kernel - Re: [PATCH v19 058/130] KVM: x86/mmu: Add a private pointer to struct kvm_mmu

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Message-ID: <ada65e3e977c8cde0044b7fa9de5f918e3b1b638.camel@intel.com>
Date: Thu, 14 Mar 2024 21:39:34 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Yuan, Hang" <hang.yuan@...el.com>,
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Chen, Bo2"
	<chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 058/130] KVM: x86/mmu: Add a private pointer to struct
 kvm_mmu_page

On Fri, 2024-03-15 at 10:23 +1300, Huang, Kai wrote:
> We have 3 page tables as you mentioned:
> 
> PT: page table
> - Shared PT is visible to KVM and it is used by CPU.
> - Private PT is used by CPU but it is invisible to KVM.
> - Dummy PT is visible to KVM but not used by CPU.  It is used to
>    propagate PT change to the actual private PT which is used by CPU.
> 
> If I recall correctly, we used to call the last one "mirrored
> (private) 
> page table".
> 
> I lost the tracking when we changed to use "dummy page table", but it
> seems to me "mirrored" is better than "dummy" because the latter
> means 
> it is useless but in fact it is used to propagate changes to the real
> private page table used by hardware.

Mirrored makes sense to me. So like:

Private - Table actually mapping private alias, in TDX module
Shared - Shared alias table, visible in KVM
Mirror - Mirroring private, visible in KVM

> 
> Btw, one nit, perhaps:
> 
> "Shared PT is visible to KVM and it is used by CPU." -> "Shared PT is
> visible to KVM and it is used by CPU for shared mappings".
> 
> To make it more clearer it is used for "shared mappings".
> 
> But this may be unnecessary to others, so up to you.

Yep, this seems clearer.