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

[Xiaoyao Li]

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

Cc: Fuad Tabba <ta...@google.com>
Cc: Xiaoyao Li <xiaoyao...@intel.com>
Signed-off-by: Sean Christopherson <sea...@google.com>
Co-developed-by: Ackerley Tng <ackerley...@google.com>
Signed-off-by: Ackerley Tng <ackerley...@google.com>

The names will finalized eventually and it won't impact the 
implementation. So,

Reviewed-by: Xiaoyao Li <xiaoyao...@intel.com>

---
  include/linux/kvm_host.h |  6 ++++++
  virt/kvm/guest_memfd.c   | 26 ++++++++++++++++++++++++++
  virt/kvm/kvm_main.c      | 12 +++++++++++-
  3 files changed, 43 insertions(+), 1 deletion(-)

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;
+
  typedef bool (kvm_mem_is_private_t)(struct kvm *kvm, gfn_t gfn);
  DECLARE_STATIC_CALL(__kvm_mem_is_private, kvm_mem_is_private_t);
  
@@ -2588,6 +2590,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
        return static_call(__kvm_mem_is_private)(kvm, gfn);
  }
  #else
+#define gmem_in_place_conversion false
+
  static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
  {
        return false;
@@ -2595,6 +2599,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, 
gfn_t gfn)
  #endif /* kvm_arch_has_private_mem */
  
  #ifdef CONFIG_KVM_GUEST_MEMFD
+bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn);
+
  int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
                     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
                     int *max_order);
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 1f1ae61b4878b..75979c885e030 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -475,6 +475,32 @@ static int kvm_gmem_mmap(struct file *file, struct 
vm_area_struct *vma)
        return 0;
  }
  
+bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn)
+{
+       struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+       struct inode *inode;
+
+       if (!slot)
+               return 0;
+
+       CLASS(gmem_get_file, file)(slot);
+       if (!file)
+               return 0;
+
+       inode = file_inode(file);
+
+       /*
+        * Rely on the maple tree's internal RCU lock to ensure a stable result.
+        * This result can become stale as soon as the lock is dropped, so the
+        * caller _must_ protect consumption of private vs. shared either by
+        * holding guest_memfd's invalidate lock for the entire duration, or by
+        * checking mmu_invalidate_retry_gfn() under mmu_lock to serialize
+        * against concurrent attribute updates.
+        */
+       return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn));
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private);
+
  static struct file_operations kvm_gmem_fops = {
        .mmap           = kvm_gmem_mmap,
        .open           = generic_file_open,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a01d336eab18b..0a5a1bee8de0f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -102,6 +102,10 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink);
  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;
+#endif
+
  /*
   * Ordering of locks:
   *
@@ -2423,6 +2427,9 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
  static u64 kvm_supported_vm_mem_attributes(struct kvm *kvm)
  {
  #ifdef kvm_arch_has_private_mem
+       if (gmem_in_place_conversion)
+               return 0;
+
        if (!kvm || kvm_arch_has_private_mem(kvm))
                return KVM_MEMORY_ATTRIBUTE_PRIVATE;
  #endif
@@ -2634,8 +2641,11 @@ EXPORT_STATIC_CALL_GPL(__kvm_mem_is_private);
  
  static void kvm_init_memory_attributes(void)
  {
+       if (gmem_in_place_conversion)
+               static_call_update(__kvm_mem_is_private, kvm_gmem_is_private);
  #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
-       static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
+       else
+               static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
  #endif
  }
  #else