Re: [PATCH v12 23/45] KVM: Let userspace disable per-VM mem attributes, enable per-gmem attributes

[Ackerley Tng]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Tue, Sep 01, 2026, Fuad Tabba wrote:
>> Hi Sean, Ackerley,
>>
>> On Mon, 31 Aug 2026 at 01:25, Ackerley Tng via B4 Relay
>> <devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>> ...
>> > +       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, 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).
>>
>> The parameter is only settable when CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y, and
>> in that config gmem_in_place_conversion initialises to
>> !IS_ENABLED(CONFIG_KVM_VM_MEMORY_ATTRIBUTES), i.e. N, not Y. Is the doc line
>> wrong, or is N-by-default not what was intended here?
>
> The doc is wrong.  The intent is to maintain backwards compatibility with existing
> deployments if KVM_VM_MEMORY_ATTRIBUTES=y.
>

Thanks Fuad! I got confused, gmem_in_place_conversion the variable
defaults to true (because CONFIG_KVM_VM_MEMORY_ATTRIBUTES is by default
not set), but if the parameter exists, CONFIG_KVM_VM_MEMORY_ATTRIBUTES
must be set and so the parameter defaults to N.

Will fix in the next revision.

>> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> > index 05c518c9b8078..929fd3e1a01e6 100644
>> > --- a/virt/kvm/kvm_main.c
>> > +++ b/virt/kvm/kvm_main.c
>> > @@ -103,7 +103,10 @@ 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;
>> > +bool __ro_after_init gmem_in_place_conversion = !IS_ENABLED(CONFIG_KVM_VM_MEMORY_ATTRIBUTES);
>> > +#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
>> > +module_param(gmem_in_place_conversion, bool, 0444);
>> > +#endif
>> >  EXPORT_SYMBOL_FOR_KVM_INTERNAL(gmem_in_place_conversion);
>> >  #endif