Re: [PATCH v10 07/41] KVM: guest_memfd: Stub in ability to enable in-place shared<=>private conversion

On 8/8/2026 5:52 AM, Ackerley Tng via B4 Relay wrote:
> From: Sean Christopherson <[email protected]>
> 
> Stub in global variable to enable in-place guest_memfd private<=>shared
> memory conversion, which will eventually be exposed to userspace via a
> module param, and wire up the __kvm_mem_is_private() static call to the
> guest_memfd version when in-place conversion is enabled, i.e. when gmem is
> the sole authority on private vs. shared memory.

I find this patch changes the default memory type for a @gfn.

- When memory attribute is tracked per-VM, the default memory type is 
always shared.

- when gmem_in_place_conversion is true,
   - if the @gfn has no memslot, or the memslot where the @gfn locates
     doesn't have gmem bound, the default memory type is shared,
   - otherwise, the default memory type is determined by the
     GUEST_MEMFD_FLAG_INIT_SHARED flag.

I think we should document this change.

> Cc: Fuad Tabba <[email protected]>
> Cc: Xiaoyao Li <[email protected]>
> Signed-off-by: Sean Christopherson <[email protected]>
> Co-developed-by: Ackerley Tng <[email protected]>
> Signed-off-by: Ackerley Tng <[email protected]>
> ---
>   include/linux/kvm_host.h |  6 ++++++
>   virt/kvm/guest_memfd.c   | 26 ++++++++++++++++++++++++++
>   virt/kvm/kvm_main.c      | 12 +++++++++++-
>   3 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 65fbce46b63f4..9477ecebbbced 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2580,6 +2580,8 @@ static inline bool kvm_vm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   #endif  /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
>   
>   #ifdef kvm_arch_has_private_mem
> +extern bool gmem_in_place_conversion;
> +
>   typedef bool (kvm_mem_is_private_t)(struct kvm *kvm, gfn_t gfn);
>   DECLARE_STATIC_CALL(__kvm_mem_is_private, kvm_mem_is_private_t);
>   
> @@ -2588,6 +2590,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   	return static_call(__kvm_mem_is_private)(kvm, gfn);
>   }
>   #else
> +#define gmem_in_place_conversion false
> +
>   static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   {
>   	return false;
> @@ -2595,6 +2599,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>   #endif /* kvm_arch_has_private_mem */
>   
>   #ifdef CONFIG_KVM_GUEST_MEMFD
> +bool kvm_gmem_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);
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 1f1ae61b4878b..75979c885e030 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -475,6 +475,32 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
>   	return 0;
>   }
>   
> +bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn)
> +{
> +	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
> +	struct inode *inode;
> +
> +	if (!slot)
> +		return 0;
> +
> +	CLASS(gmem_get_file, file)(slot);
> +	if (!file)
> +		return 0;
> +
> +	inode = file_inode(file);
> +
> +	/*
> +	 * Rely on the maple tree's internal RCU lock to ensure a stable result.
> +	 * This result can become stale as soon as the lock is dropped, so the
> +	 * caller _must_ protect consumption of private vs. shared either by
> +	 * holding guest_memfd's invalidate lock for the entire duration, or by
> +	 * checking mmu_invalidate_retry_gfn() under mmu_lock to serialize
> +	 * against concurrent attribute updates.
> +	 */
> +	return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn));
> +}
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private);
> +
>   static struct file_operations kvm_gmem_fops = {
>   	.mmap		= kvm_gmem_mmap,
>   	.open		= generic_file_open,
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a01d336eab18b..0a5a1bee8de0f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -102,6 +102,10 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink);
>   static bool __ro_after_init allow_unsafe_mappings;
>   module_param(allow_unsafe_mappings, bool, 0444);
>   
> +#ifdef kvm_arch_has_private_mem
> +bool __ro_after_init gmem_in_place_conversion = false;
> +#endif
> +
>   /*
>    * Ordering of locks:
>    *
> @@ -2423,6 +2427,9 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
>   static u64 kvm_supported_vm_mem_attributes(struct kvm *kvm)
>   {
>   #ifdef kvm_arch_has_private_mem
> +	if (gmem_in_place_conversion)
> +		return 0;
> +
>   	if (!kvm || kvm_arch_has_private_mem(kvm))
>   		return KVM_MEMORY_ATTRIBUTE_PRIVATE;
>   #endif
> @@ -2634,8 +2641,11 @@ EXPORT_STATIC_CALL_GPL(__kvm_mem_is_private);
>   
>   static void kvm_init_memory_attributes(void)
>   {
> +	if (gmem_in_place_conversion)
> +		static_call_update(__kvm_mem_is_private, kvm_gmem_is_private);
>   #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
> -	static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
> +	else
> +		static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
>   #endif
>   }
>   #else
>