[PATCH RFC v3 29/88] KVM: guest_memfd: Support getting PFNs for stage 2 from providers

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Ackerley Tng <ackerleytng@google.com>

Add .get_pfn callback to struct guest_memfd_provider_operations to
support physical memory providers (e.g. devices, DAX, or pre-allocated
ranges) that supply PFNs directly without folio allocation or page cache
insertion.

Define GUEST_MEMFD_FLAG_PFN_MODE in uapi/linux/kvm.h and define
kvm_gmem_is_pfn_mode() to check for this flag in gi->flags.

In __kvm_gmem_get_pfn(), query .get_pfn() directly when in PFN mode
and return NULL on success (instead of a struct folio *).

Note that .alloc_folio is still required by kvm_gmem_attach_resource() and
GUEST_MEMFD_FLAG_PFN_MODE is not yet enabled for userspace.  Subsequent
patches will update the rest of guest_memfd (e.g. mmap, fallocate,
truncation, and attributes) to support PFN mode before enabling the flag
and allowing external PFN-backed providers to be attached.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 include/linux/guest_memfd.h | 11 +++++++++++
 include/uapi/linux/kvm.h    |  1 +
 virt/kvm/guest_memfd.c      | 48 ++++++++++++++++++++++++++++++++++++++++-----
 3 files changed, 55 insertions(+), 5 deletions(-)

diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
index df2db2a0bae6e..9fdc0bd1a7190 100644
--- a/include/linux/guest_memfd.h
+++ b/include/linux/guest_memfd.h
@@ -20,6 +20,15 @@ struct mempolicy;
  *               Must return a pointer to a struct folio on success, or an
  *               ERR_PTR() error code on failure. Must never return NULL.
  * @invalidate_folio: Notify provider that a folio has been invalidated.
+ * @get_pfn: Obtain a PFN for a given index. The provider must ensure backing
+ *           memory is present and is responsible for its contents, such as
+ *           zeroing if necessary. On success, return 0 with the physical frame
+ *           number in @pfn and, if non-NULL, the PFN attributes in @pfn_attrs
+ *           and the maximum page order of contiguous backing memory containing
+ *           @pfn with uniform @pfn_attrs in @max_order. Once @get_pfn succeeds,
+ *           the provider must keep the @index-to-@pfn mapping and @pfn_attrs
+ *           constant, and the memory accessible, until guest_memfd stops using
+ *           this page. Returns a negative error code on failure.
  *
  * Used by filesystems and device drivers that provide memory for guest_memfd.
  */
@@ -29,6 +38,8 @@ struct guest_memfd_provider_operations {
 	struct folio *(*alloc_folio)(void *provider, pgoff_t index,
 				     struct mempolicy *mpol);
 	void (*invalidate_folio)(void *provider, struct folio *folio);
+	int (*get_pfn)(void *provider, pgoff_t index, unsigned long *pfn,
+		       int *max_order, u64 *pfn_attrs);
 };
 
 #endif /* _LINUX_GUEST_MEMFD_H */
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index f4108bf7e3192..6798056f50e32 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1675,6 +1675,7 @@ struct kvm_memory_attributes2 {
 #define GUEST_MEMFD_FLAG_MMAP		(1ULL << 0)
 #define GUEST_MEMFD_FLAG_INIT_SHARED	(1ULL << 1)
 #define GUEST_MEMFD_FLAG_USE_RESOURCE	(1ULL << 2)
+#define GUEST_MEMFD_FLAG_PFN_MODE	(1ULL << 3)
 
 struct kvm_create_guest_memfd {
 	__u64 size;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index f0bd90fd9c986..eedf03df12c55 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -54,6 +54,11 @@ static __always_inline struct gmem_inode *GMEM_I(struct inode *inode)
 	return container_of(inode, struct gmem_inode, vfs_inode);
 }
 
+static bool kvm_gmem_is_pfn_mode(struct gmem_inode *gi)
+{
+	return gi->flags & GUEST_MEMFD_FLAG_PFN_MODE;
+}
+
 static void *gmem_native_attach(struct inode *gmem_inode,
 				struct file *resource_file)
 {
@@ -124,6 +129,26 @@ static struct folio *gmem_provider_alloc_folio(struct gmem_inode *gi,
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+static int gmem_provider_get_pfn(struct gmem_inode *gi, pgoff_t index,
+				 kvm_pfn_t *pfn, int *max_order,
+				 u64 *pfn_attrs)
+{
+	unsigned long raw_pfn;
+	u64 __attrs = 0;
+	int r;
+
+	pfn_attrs = pfn_attrs ?: &__attrs;
+	*pfn_attrs = 0;
+
+	r = gi->provider_ops->get_pfn(gi->provider, index, &raw_pfn, max_order,
+				      pfn_attrs);
+	if (r)
+		return r;
+
+	*pfn = raw_pfn;
+	return 0;
+}
+
 static void gmem_provider_invalidate_folio(struct gmem_inode *gi,
 					   struct folio *folio)
 {
@@ -1270,15 +1295,17 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 	filemap_invalidate_unlock(file->f_mapping);
 }
 
-/* Returns a locked folio on success.  */
+/* On success, return a locked folio in folio mode and NULL in PFN mode. */
 static struct folio *__kvm_gmem_get_pfn(struct file *file,
 					struct kvm_memory_slot *slot,
 					pgoff_t index, kvm_pfn_t *pfn,
 					int *max_order, u64 *pfn_attrs)
 {
 	struct file *slot_file = READ_ONCE(slot->gmem.file);
+	struct gmem_inode *gi = GMEM_I(file_inode(file));
 	struct gmem_file *f = file->private_data;
 	struct folio *folio;
+	int r;
 
 	if (file != slot_file) {
 		WARN_ON_ONCE(slot_file);
@@ -1290,6 +1317,13 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
 		return ERR_PTR(-EIO);
 	}
 
+	if (kvm_gmem_is_pfn_mode(gi)) {
+		r = gmem_provider_get_pfn(gi, index, pfn, max_order, pfn_attrs);
+		if (r)
+			return ERR_PTR(r);
+		return NULL;
+	}
+
 	folio = kvm_gmem_get_folio(file_inode(file), index);
 	if (IS_ERR(folio))
 		return folio;
@@ -1355,8 +1389,10 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 					       (kvm_pfn_t)1 << *max_order);
 
 out_put_folio:
-	folio_unlock(folio);
-	folio_put(folio);
+	if (folio) {
+		folio_unlock(folio);
+		folio_put(folio);
+	}
 
 out:
 	filemap_invalidate_unlock_shared(file_inode(file)->i_mapping);
@@ -1384,7 +1420,8 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
 		goto out_unlock;
 	}
 
-	folio_unlock(folio);
+	if (folio)
+		folio_unlock(folio);
 
 	/* Populating pages initializes private memory, which cannot be read-only. */
 	if (pfn_attrs & GUEST_MEMFD_PFN_ATTRS_READONLY) {
@@ -1400,7 +1437,8 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
 	ret = post_populate(kvm, gfn, pfn, src_page, opaque);
 
 out_put_folio:
-	folio_put(folio);
+	if (folio)
+		folio_put(folio);
 out_unlock:
 	filemap_invalidate_unlock(file->f_mapping);
 	return ret;

-- 
2.56.0.455.g6c5a503b38-goog