Re: [PATCH v4 04/17] KVM: x86/mmu: Prevent huge page promotion for mirror roots in fault path

[Edgecombe, Rick P]

On Mon, 2026-09-28 at 17:09 +0800, Yan Zhao wrote:
> From: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> 
> Disallow huge page promotion in the TDP MMU for mirror roots as KVM doesn't
> currently support promoting S-EPT entries due to the complexity incurred
> by the TDX module's rules for huge page promotion.
> 
>  - The current TDX module requires all 4KB leafs to be either all PENDING
>    or all ACCEPTED before a successful promotion to 2MB. This requirement
>    prevents successful page merging after partially converting a 2MB
>    range from private to shared and then back to private, which is the
>    primary scenario necessitating page promotion.
> 
>  - The TDX module effectively requires a break-before-make sequence (to
>    satisfy its TLB flushing rules), i.e., creates a window of time where a
>    different vCPU can encounter faults on a SPTE that KVM is trying to
>    promote to a huge page.  To avoid unexpected BUSY errors, KVM would need
>    to FREEZE the non-leaf SPTE before replacing it with a huge SPTE.
> 
> Disable huge page promotion for all map() operations, as supporting page
> promotion when building the initial image is still non-trivial, and the
> vast majority of images are ~4MB or less, i.e., the benefit of creating
> huge pages during TD build time is minimal.
> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> [sean: check root, add comment, rewrite changelog]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> ---

LGTM. I'm listed as the patch author I think because I suggested it via diff,
but due to the longish SOB chain after here is this too:

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>