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

On 8/24/2026 10:45 PM, Ackerley Tng wrote:
> Xiaoyao Li <[email protected]> writes:
> 
>> 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.
>>
> 
> In a way it's not really a "change" since gmem_in_place_conversion is
> set up as false in this patch.

I agree. It is not this patch making the change but the patch allows 
gmem_in_place_conversion to be true.

> 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

I think the default is disabled?

> 			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. 

I start to think that the term "in-place conversion" seems to read 
inaccurate. I think it is describing the shared/private conversion of a 
gfn, and in-place means when a gfn is converted between shared/private, 
the backend comes from the same gmem page, thus in-place. But KVM 
doesn't enforce the "in-place".

If "in-place conversion" describes the shared/private conversion of a 
gmem page, then "in-place" is redundant because the conversion a 
specific gmem page is always in-place.

>                       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).

I'm looking at the doc of KVM_SET_USER_MEMORY_REGION2, which reads

# When mapping a gfn into the guest, KVM selects shared vs. private, i.e 
consumes
# userspace_addr vs. guest_memfd, based on the gfn's 
KVM_MEMORY_ATTRIBUTE_PRIVATE
# state.  At VM creation time, all memory is shared, i.e. the PRIVATE 
attribute
# is '0' for all gfns.  Userspace can control whether memory is 
shared/private by
# toggling KVM_MEMORY_ATTRIBUTE_PRIVATE via KVM_SET_MEMORY_ATTRIBUTES as 
needed.