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

[Isaku Yamahata]

On Thu, Mar 14, 2024 at 09:39:34PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx> wrote:

> 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.

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
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@xxxxxxxxx>