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

Binbin Wu
2024-03-27T13:49:21Z

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.
>