Re: [PATCH v8 06/13] KVM: x86: Generalize private fault lookups to guest_memfd fault lookups

[James Houghton <jthoughton@google.com>]

On Tue, May 6, 2025 at 1:47 PM Ackerley Tng <ackerleytng@google.com> wrote:
> From here [1], these changes will make it to v9
>
> + kvm_max_private_mapping_level renaming to kvm_max_gmem_mapping_level
> + kvm_mmu_faultin_pfn_private renaming to kvm_mmu_faultin_pfn_gmem
>
> > Only kvm_mmu_hugepage_adjust() must be taught to not rely on
> > fault->is_private.
> >
>
> I think fault->is_private should contribute to determining the max
> mapping level.
>
> By the time kvm_mmu_hugepage_adjust() is called,
>
> * For Coco VMs using guest_memfd only for private memory,
>   * fault->is_private would have been checked to align with
>     kvm->mem_attr_array, so
> * For Coco VMs using guest_memfd for both private/shared memory,
>   * fault->is_private would have been checked to align with
>     guest_memfd's shareability
> * For non-Coco VMs using guest_memfd
>   * fault->is_private would be false

I'm not sure exactly which thread to respond to, but it seems like the
idea now is to have a *VM* flag determine if shared faults use gmem or
use the user mappings. It seems more natural for that to be a property
of the memslot / a *memslot* flag.

Sean, Fuad, what do you think? I don't see any downsides, and it seems
strictly more flexible.

> [1] https://lore.kernel.org/all/20250430165655.605595-7-tabba@google.com/
> [2] https://lore.kernel.org/all/diqz1pt1sfw8.fsf@ackerleytng-ctop.c.googlers.com/