Re: [RFC PATCH 02/12] KVM: x86/mmu: Add dedicated API to map guest_memfd pfn into TDP MMU

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 28, 2025, Yan Zhao wrote:
> On Thu, Aug 28, 2025 at 09:26:50AM +0800, Edgecombe, Rick P wrote:
> > On Wed, 2025-08-27 at 17:54 -0700, Rick Edgecombe wrote:
> > > > 
> > > > Then, what about setting
> > > >                 .max_level = PG_LEVEL_4K,
> > > > directly?
> > > > 
> > > > Otherwise, the "(KVM_BUG_ON(level != PG_LEVEL_4K, kvm)" would be triggered
> > > > in
> > > > tdx_sept_set_private_spte().
> > > 
> > > Yes this fails to boot a TD. With max_level = PG_LEVEL_4K it passes the full
> > > tests. I don't think it's ideal to encode PAGE.ADD details here though.
> > > 
> > > But I'm not immediately clear what is going wrong. The old struct
> > > kvm_page_fault
> > > looks pretty similar. Did you root cause it?
> >
> > Oh, duh. Because we are passing in the PFN now so it can't know the size. So
> > it's not about PAGE.ADD actually.
> Right, it's because the previous kvm_tdp_map_page() updates fault->max_level in
> kvm_mmu_faultin_pfn_private() by checking the private_max_mapping_level hook.
> 
> However, private_max_mapping_level() skips the faultin step and goes straight
> to kvm_tdp_mmu_map().
> 
> > Sill, how about calling the function kvm_tdp_mmu_map_private_pfn_4k(), or
> > passing in the level?
> Looks [1] can also address this issue. Not sure which one Sean prefers.
> 
> [1] https://lore.kernel.org/all/20250729225455.670324-15-seanjc@google.com

That won't fix this issue though, becuase @fault will be valid and so max_level
will still be KVM_MAX_HUGEPAGE_LEVEL.  Which is by design, the intent in that
flow is that KVM should have gotten the level when getting the pfn from gmem.

IIUC, this particular flow _must_ map at 4KiB, so I think forcing PG_LEVEL_4k is
the right solution.