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

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Message-ID: <35090f7e-4f4d-403c-b95e-f09248fc272d@linux.intel.com>
Date: Wed, 27 Mar 2024 21:49:14 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: "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 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.

> 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.
>