Re: [PATCH v3 11/16] KVM: TDX: Add x86 ops for external spt cache

Kiryl Shutsemau
2025-09-19T09:44:49Z

On Thu, Sep 18, 2025 at 04:22:19PM -0700, Rick Edgecombe wrote:
> Move mmu_external_spt_cache behind x86 ops.
>
> In the mirror/external MMU concept, the KVM MMU manages a non-active EPT
> tree for private memory (the mirror). The actual active EPT tree the
> private memory is protected inside the TDX module. Whenever the mirror EPT
> is changed, it needs to call out into one of a set of x86 opts that
> implement various update operation with TDX specific SEAMCALLs and other
> tricks. These implementations operate on the TDX S-EPT (the external).
>
> In reality these external operations are designed narrowly with respect to
> TDX particulars. On the surface, what TDX specific things are happening to
> fulfill these update operations are mostly hidden from the MMU, but there
> is one particular area of interest where some details leak through.
>
> The S-EPT needs pages to use for the S-EPT page tables. These page tables
> need to be allocated before taking the mmu lock, like all the rest. So the
> KVM MMU pre-allocates pages for TDX to use for the S-EPT in the same place
> where it pre-allocates the other page tables. It’s not too bad and fits
> nicely with the others.
>
> However, Dynamic PAMT will need even more pages for the same operations.
> Further, these pages will need to be handed to the arch/86 side which used
> them for DPAMT updates, which is hard for the existing KVM based cache.
> The details living in core MMU code start to add up.
>
> So in preparation to make it more complicated, move the external page
> table cache into TDX code by putting it behind some x86 ops. Have one for
> topping up and one for allocation. Don’t go so far to try to hide the
> existence of external page tables completely from the generic MMU, as they
> are currently stores in their mirror struct kvm_mmu_page and it’s quite
> handy.
>
> To plumb the memory cache operations through tdx.c, export some of
> the functions temporarily. This will be removed in future changes.
>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Makes sense.

Acked-by: Kiryl Shutsemau <kas@kernel.org>

--
Kiryl Shutsemau / Kirill A. Shutemov