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

Sean Christopherson [off-list ref] writes:

On Wed, Aug 12, 2026, David Hildenbrand (Arm) wrote:

quoted

On 8/10/26 17:01, Sean Christopherson wrote:

quoted

On Mon, Aug 10, 2026, David Hildenbrand (Arm) wrote:

quoted

On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote:

quoted

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;

Is there a "supports/has/enable" in there? And should we call it "kvm_gmem" for
completeness?

It's kinda stupid and definitely more than a bit inconsistent, but overall I think
I actually like "gmem_in_place_conversion" the best.

gmem_has_in_place_conversion and gmem_supports_in_place_conversion are misleading
because it's not just that guest_memfd has/supports in-place conversion, it's that
that KVM is tracking PRIVATE in guest_memfd and so in-place conversion is the only
option.

On the other hand, while gmem_in_place_conversion_enabled is better, it's not
quite accurate either because userspace isn't strictly required to do in-place
conversion.

As for a kvm_ prefix, IMO gmem_ is sufficient for a namespace, and not having kvm_
is consistent with most module params in KVM.

Maybe we should have a helper function instead of accessing the module parameter
directly, then the gmem_in_place_conversion could just stay file-local and
kvm_gmem_in_place_conversion() would be used by other code that wants to obtain
the value.

Instead of the

#define mem_in_place_conversion false

We'd have

#define kvm_gmem_in_place_conversion()	false

just a thought ...

FWIW, I'd rather prefix kvm_ than add a wrapper to get a boolean.

Could you elaborate on the rationale for a wrapper? I don't really see a
reason to encapsulate this boolean since this flag should be global to
KVM, it toggles between tracking shared/private status at the VM or
guest_memfd level. Since KVM is a module I guess it's already global
just to KVM?

On the naming, Sashiko did point out somewhere that
gmem_in_place_conversion as a global might end up shadowing or being
shadowed by something else.

Prefixing kvm_ was one of Sashiko's suggestions too.

I don't really have a strong opinion on this but I'd like to know what
to put in v11.

Will using

  module_param_named(gmem_in_place_conversion,
kvm_gmem_in_place_conversion, bool, 0444);

help? The module param doesn't have the kvm prefix but the variable
does.