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

[Ackerley Tng]

Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:

> On 8/8/2026 5:52 AM, Ackerley Tng via B4 Relay wrote:
>> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>>
>> 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.
>

In a way it's not really a "change" since gmem_in_place_conversion is
set up as false in this patch.

When it is enabled in a later patch, another way to see it is that the
default remains "shared unless defined as private".

Before:

+ no memslot, or memslot not bound to gmem => shared
+ memory is shared unless VM ioctl used to make gfn private.

After:

+ no memslot, or memslot not bound to gmem => still shared
+ otherwise, ask gmem about status, which I think is already captured in
  the module param concept.
    + The very usage of gmem (without INIT_SHARED) is defining memory as
      private, I think that is already documented elsewhere when
      INIT_SHARED was introduced, that now the default is private.

So in summary, I feel that this has already been documented in various
places. I'll also add the following in v11's "KVM: Let userspace disable
per-VM mem attributes, enable per-gmem attributes", in
Documentation/admin-guide/kernel-parameters.txt:

	kvm.gmem_in_place_conversion=
			[KVM] Controls whether KVM enables in-place conversion
			support for guest_memfd and tracks the private/shared
			state of memory per guest_memfd instead of per VM.

			If enabled (the default), KVM enables the
			KVM_SET_MEMORY_ATTRIBUTES2 ioctl on guest_memfd file
			descriptors and disables the legacy VM-scoped
			KVM_SET_MEMORY_ATTRIBUTES ioctl for private memory state
			tracking. Only the KVM_MEMORY_ATTRIBUTE_PRIVATE
			attribute moves to per-guest_memfd tracking; other
			attributes remain per-VM.

			This parameter toggles KVM's in-place conversion
			capability support. Whether a VMM uses separate backends
			or out-of-place memory management is determined by
			userspace VMM design.

			Note, this parameter is only available when
			CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y. When
			CONFIG_KVM_VM_MEMORY_ATTRIBUTES is not set, in-place
			conversion is unconditionally enabled.

			Default is Y (on).


>>
>> [...snip...]
>>