Re: [PATCH v19 058/130] KVM: x86/mmu: Add a private pointer to struct kvm_mmu_page

[Binbin Wu]

On 3/28/2024 8:02 AM, Isaku Yamahata wrote:
> On Wed, Mar 27, 2024 at 09:49:14PM +0800,
> Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> 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.