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

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Date: Wed, 27 Mar 2024 17:02:05 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Isaku Yamahata <isaku.yamahata@...el.com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"Huang, Kai" <kai.huang@...el.com>,
	"Zhang, Tina" <tina.zhang@...el.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"Yuan, Hang" <hang.yuan@...el.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 Wed, Mar 27, 2024 at 09:49:14PM +0800,
Binbin Wu <binbin.wu@...ux.intel.com> wrote:

> 
> 
> On 3/15/2024 9:09 AM, Isaku Yamahata wrote:
> > Here is the updated one. Renamed dummy -> mirroed.
> > 
> > When KVM resolves the KVM page fault, it walks the page tables.  To reuse
> > the existing KVM MMU code and mitigate the heavy cost of directly walking
> > the private page table, allocate one more page to copy the mirrored page
> 
> Here "copy" is a bit confusing for me.
> The mirrored page table is maintained by KVM, not copied from anywhere.

How about, "maintain" or "keep"?

> 
> > table for the KVM MMU code to directly walk.  Resolve the KVM page fault
> > with the existing code, and do additional operations necessary for the
> > private page table.  To distinguish such cases, the existing KVM page table
> > is called a shared page table (i.e., not associated with a private page
> > table), and the page table with a private page table is called a mirrored
> > page table.  The relationship is depicted below.
> > 
> > 
> >                KVM page fault                     |
> >                       |                           |
> >                       V                           |
> >          -------------+----------                 |
> >          |                      |                 |
> >          V                      V                 |
> >       shared GPA           private GPA            |
> >          |                      |                 |
> >          V                      V                 |
> >      shared PT root      mirrored PT root         |    private PT root
> >          |                      |                 |           |
> >          V                      V                 |           V
> >       shared PT           mirrored PT ----propagate---->  private PT
> >          |                      |                 |           |
> >          |                      \-----------------+------\    |
> >          |                                        |      |    |
> >          V                                        |      V    V
> >    shared guest page                              |    private guest page
> >                                                   |
> >                             non-encrypted memory  |    encrypted memory
> >                                                   |
> > PT: Page table
> > Shared PT: visible to KVM, and the CPU uses it for shared mappings.
> > Private PT: the CPU uses it, but it is invisible to KVM.  TDX module
> >              updates this table to map private guest pages.
> > Mirrored PT: It is visible to KVM, but the CPU doesn't use it.  KVM uses it
> >               to propagate PT change to the actual private PT.
> > 
> 
> 

-- 
Isaku Yamahata <isaku.yamahata@...el.com>