From: Sean Christopherson <seanjc@google.com>

Implement kvm_gmem_get_memory_attributes() for guest_memfd to allow the KVM
core and architecture code to query per-GFN memory attributes.

kvm_gmem_get_memory_attributes() finds the memory slot for a given GFN and
queries the guest_memfd file's to determine if the page is marked as
private.

If vm_memory_attributes is not enabled, there is no shared/private tracking
at the VM level. Install the guest_memfd implementation as long as
guest_memfd is enabled to give guest_memfd a chance to respond on
attributes.

guest_memfd should look up attributes regardless of whether this memslot is
gmem-only since attributes are now tracked by gmem regardless of whether
mmap() is enabled.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Ackerley Tng <redacted>
Signed-off-by: Ackerley Tng <redacted>
---
  include/linux/kvm_host.h |  2 ++
  virt/kvm/guest_memfd.c   | 29 +++++++++++++++++++++++++++++
  virt/kvm/kvm_main.c      |  3 +++
  3 files changed, 34 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 512febf47c265..b8418cc5851f1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h

@@ -2543,6 +2543,8 @@ bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
  					 struct kvm_gfn_range *range);
  #endif /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
  
+unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn);
+
  #ifdef CONFIG_KVM_GUEST_MEMFD
  int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
  		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 26cec833766c3..f62facc3ab776 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c

@@ -518,6 +518,35 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
  	return 0;
  }
  
+unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
+{
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+
+	/*
+	 * If this gfn has no associated memslot, there's no chance of the gfn
+	 * being backed by private memory, since guest_memfd must be used for
+	 * private memory, and guest_memfd must be associated with some memslot.
+	 */
+	if (!slot)
+		return 0;
+
+	CLASS(gmem_get_file, file)(slot);
+	if (!file)
+		return false;
+
+	/*
+	 * Don't take the filemap invalidation lock, as temporarily acquiring
+	 * that lock wouldn't provide any meaningful protection.  The caller
+	 * _must_ protect consumption of private vs. shared by checking
+	 * mmu_invalidate_retry_gfn() under mmu_lock.
+	 */
+	guard(rcu)();
+
+	return kvm_gmem_get_attributes(file_inode(file),
+				       kvm_gmem_get_index(slot, gfn));
+}
+EXPORT_SYMBOL_GPL(kvm_gmem_get_memory_attributes);
+
  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 6c29770dfa7c8..c73ebdb73070e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -2660,6 +2660,9 @@ static void kvm_init_memory_attributes(void)
  	if (vm_memory_attributes)
  		static_call_update(__kvm_get_memory_attributes,
  				   kvm_get_vm_memory_attributes);
+	else if (IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
+		static_call_update(__kvm_get_memory_attributes,
+				   kvm_gmem_get_memory_attributes);
  	else
  		static_call_update(__kvm_get_memory_attributes,
  				   (void *)__static_call_return0);