Re: [PATCH v3 06/24] KVM: x86/mmu: Disallow page merging (huge page adjustment) for mirror root

[Yan Zhao <yan.y.zhao@intel.com>]

Hi Sean,
Thanks for the review!

On Thu, Jan 15, 2026 at 02:49:59PM -0800, Sean Christopherson wrote:
> On Tue, Jan 06, 2026, Yan Zhao wrote:
> > From: Rick P Edgecombe <rick.p.edgecombe@intel.com>
> > 
> > Disallow page merging (huge page adjustment) for the mirror root by
> > utilizing disallowed_hugepage_adjust().
> 
> Why?  What is this actually doing?  The below explains "how" but I'm baffled as
> to the purpose.  I'm guessing there are hints in the surrounding patches, but I
> haven't read them in depth, and shouldn't need to in order to understand the
> primary reason behind a change.
Sorry for missing the background. I will explain the "why" in the patch log in
the next version.

The reason for introducing this patch is to disallow page merging for TDX. I
explained the reasons to disallow page merging in the cover letter:

"
7. Page merging (page promotion)

   Promotion is disallowed, because:

   - 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.

   - tdh_mem_page_promote() depends on tdh_mem_range_block() in the current
     TDX module. Consequently, handling BUSY errors is complex, as page
     merging typically occurs in the fault path under shared mmu_lock.

   - Limited amount of initial private memory (typically ~4MB) means the
     need for page merging during TD build time is minimal.
"

Without this patch, page promotion may be triggered in the following scenario:

1. guest_memfd allocates a 2MB folio for GPA X, so the max mapping level is 2MB.
2. KVM maps GPA X at 4KB level during TD build time.
3. Guest converts GPA X to shared, zapping the 4KB leaf private mapping while
   keeping the 2MB non-leaf private mapping.
3. Guest converts GPA X to private and accepts it at 2MB level.
4. KVM maps GPA X at 2MB level, triggering page merging.

However, we currently don't support page merging yet. Specifically for the above
scenario, the purpose is to avoid handling the error from
tdh_mem_page_promote(), which SEAMCALL currently needs to be preceded by
tdh_mem_range_block(). To handle the promotion error (e.g., due to busy) under
read mmu_lock, we may need to introduce several spinlocks and guarantees from
the guest to ensure the success of tdh_mem_range_unblock() to restore the S-EPT
status. 

Therefore, we introduced this patch for simplicity, and because the promotion
scenario is not common.