Re: [PATCH v11 11/18] KVM: x86: Consult guest_memfd when computing max_mapping_level

[David Hildenbrand <david@redhat.com>]

On 05.06.25 17:37, Fuad Tabba wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 
> This patch adds kvm_gmem_max_mapping_level(), which always returns
> PG_LEVEL_4K since guest_memfd only supports 4K pages for now.
> 
> When guest_memfd supports shared memory, max_mapping_level (especially
> when recovering huge pages - see call to __kvm_mmu_max_mapping_level()
> from recover_huge_pages_range()) should take input from
> guest_memfd.
> 
> Input from guest_memfd should be taken in these cases:
> 
> + if the memslot supports shared memory (guest_memfd is used for
>    shared memory, or in future both shared and private memory) or
> + if the memslot is only used for private memory and that gfn is
>    private.
> 
> If the memslot doesn't use guest_memfd, figure out the
> max_mapping_level using the host page tables like before.
> 
> This patch also refactors and inlines the other call to
> __kvm_mmu_max_mapping_level().
> 
> In kvm_mmu_hugepage_adjust(), guest_memfd's input is already
> provided (if applicable) in fault->max_level. Hence, there is no need
> to query guest_memfd.
> 
> lpage_info is queried like before, and then if the fault is not from
> guest_memfd, adjust fault->req_level based on input from host page
> tables.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Co-developed-by: Fuad Tabba <tabba@google.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---

[...]

>   static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   {
>   	return false;
> @@ -2561,6 +2565,7 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>   		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
>   		     int *max_order);
> +int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot, gfn_t gfn);
>   #else
>   static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>   				   struct kvm_memory_slot *slot, gfn_t gfn,
> @@ -2570,6 +2575,12 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>   	KVM_BUG_ON(1, kvm);
>   	return -EIO;
>   }
> +static inline int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot,
> +					 gfn_t gfn)
> +{
> +	BUG();
> +	return 0;

As raised, no BUG(). If this is unreachable for these configs,

BUILD_BUG() might do.

Apart from that

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb