Re: [PATCH v15 12/21] KVM: x86/mmu: Consult guest_memfd when computing max_mapping_level

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/18/2025 1:32 PM, Xiaoyao Li wrote:
> On 7/18/2025 12:27 AM, Fuad Tabba wrote:
>> From: Ackerley Tng <ackerleytng@google.com>
>>
>> Modify kvm_mmu_max_mapping_level() to consult guest_memfd for memory
>> regions backed by it when computing the maximum mapping level,
>> especially during huge page recovery.
> 
> IMHO, we need integrate the consultation of guest_memfd into 
> __kvm_mmu_max_mapping_level, not kvm_mmu_max_mapping_level().
> 
> __kvm_mmu_max_mapping_level() (called by kvm_mmu_hugepage_adjust()) is 
> the function KVM X86 uses to determine the final mapping level,
> fault->goal_level.
> 

I think I can understand the patch now.

For normal TDP page fault that requires KVM to setup the TDP page table 
to map the guest memory. The max page level of guest memfd is consulted 
when faulting in the pfn in kvm_mmu_faultin_pfn_private() and update 
fault->max_level accordingly. So skip consultation in 
__kvm_mmu_max_mapping_level() is OK.

But for recover_huge_pages_range() and kvm_mmu_zap_collapsible_spte() 
(this patch misses this case) which call kvm_mmu_max_mapping_level() and 
without information of the max page level of guest memfd. So we need to 
consult guest memfd separately.

But the changelog doesn't clarify it such way.