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

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Message-ID: <d6285364-f6cd-49e6-a438-2ae87f965e4f@linux.intel.com>
Date: Thu, 28 Mar 2024 11:17:45 +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 3/28/2024 8:02 AM, Isaku Yamahata wrote:
> 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"?

Or just use "for"?

i.e, allocate one more page for the mirrored page table ...



>
>>> 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.
>>>
>>