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

Isaku Yamahata
2024-03-28T00:02:06Z

On Wed, Mar 27, 2024 at 09:49:14PM +0800,
Binbin Wu <binbin.wu@linux.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@intel.com>