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

[Fuad Tabba]

On Tue, 1 Sept 2026 at 19:26, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> 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.

That explains it. Thanks.
/fuad

>
> > > 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