[PATCH v5 35/36] KVM: guest_memfd: bind backing memory to a NUMA node at creation

[Gregory Price <gourry@gourry.net>]

guest_memfd presently allocates its page-cache folios through a
per-inode shared mempolicy (kvm_gmem_get_folio()).

Today that policy can only be set after the fact, via mbind() on
a host mmap of the fd. This requires the fd to be mmap-able and
cannot reach folios that are only ever guest-faulted (no host VMA).

Neither holds for a non-mappable (confidential) guest_memfd.

Add GUEST_MEMFD_FLAG_BIND_NODE.

When set, KVM builds an MPOL_BIND policy for the requested node and
installs it over the whole inode's shared policy, so every folio
is allocated on the requested node with no userspace mbind().

The flag is advertised through KVM_CAP_GUEST_MEMFD_FLAGS only when
CONFIG_NUMA is enabled.

Suggested-by: Dave Jiang <dave.jiang@intel.com>
Co-developed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Gregory Price <gourry@gourry.net>
Assisted-by: Claude:claude-opus-4-8
---
 include/linux/kvm_host.h |  3 +++
 include/uapi/linux/kvm.h |  5 ++++-
 virt/kvm/guest_memfd.c   | 39 +++++++++++++++++++++++++++++++++++++--
 3 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ab8cfaec82d31..f6448a73d464c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -739,6 +739,9 @@ static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
 	if (!kvm || kvm_arch_supports_gmem_init_shared(kvm))
 		flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
 
+	if (IS_ENABLED(CONFIG_NUMA))
+		flags |= GUEST_MEMFD_FLAG_BIND_NODE;
+
 	return flags;
 }
 #endif
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 419011097fa8e..a8ff150e2cbb7 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1654,11 +1654,14 @@ struct kvm_memory_attributes {
 #define KVM_CREATE_GUEST_MEMFD	_IOWR(KVMIO,  0xd4, struct kvm_create_guest_memfd)
 #define GUEST_MEMFD_FLAG_MMAP		(1ULL << 0)
 #define GUEST_MEMFD_FLAG_INIT_SHARED	(1ULL << 1)
+#define GUEST_MEMFD_FLAG_BIND_NODE	(1ULL << 2)
 
 struct kvm_create_guest_memfd {
 	__u64 size;
 	__u64 flags;
-	__u64 reserved[6];
+	__u32 node;
+	__u32 pad;
+	__u64 reserved[5];
 };
 
 #define KVM_PRE_FAULT_MEMORY	_IOWR(KVMIO, 0xd5, struct kvm_pre_fault_memory)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index f0e5da4908660..de320df08e0ff 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -456,6 +456,26 @@ static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma,
 	 */
 	return mpol_shared_policy_lookup(&GMEM_I(inode)->policy, pgoff);
 }
+
+static int kvm_gmem_bind_node(struct inode *inode, int node)
+{
+	struct mempolicy *pol;
+	int err;
+
+	pol = mpol_bind_node(node);
+	if (IS_ERR(pol))
+		return PTR_ERR(pol);
+
+	err = mpol_set_shared_policy_range(&GMEM_I(inode)->policy, 0,
+					  MAX_LFS_FILESIZE >> PAGE_SHIFT, pol);
+	mpol_put(pol);
+	return err;
+}
+#else
+static int kvm_gmem_bind_node(struct inode *inode, int node)
+{
+	return -EINVAL;
+}
 #endif /* CONFIG_NUMA */
 
 static const struct vm_operations_struct kvm_gmem_vm_ops = {
@@ -557,7 +577,7 @@ bool __weak kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
 	return true;
 }
 
-static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
+static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags, int node)
 {
 	static const char *name = "[kvm-gmem]";
 	struct gmem_file *f;
@@ -598,6 +618,12 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 
 	GMEM_I(inode)->flags = flags;
 
+	if (flags & GUEST_MEMFD_FLAG_BIND_NODE) {
+		err = kvm_gmem_bind_node(inode, node);
+		if (err)
+			goto err_inode;
+	}
+
 	file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops);
 	if (IS_ERR(file)) {
 		err = PTR_ERR(file);
@@ -630,6 +656,7 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 {
 	loff_t size = args->size;
 	u64 flags = args->flags;
+	int node = NUMA_NO_NODE;
 
 	if (flags & ~kvm_gmem_get_supported_flags(kvm))
 		return -EINVAL;
@@ -637,7 +664,15 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	if (size <= 0 || !PAGE_ALIGNED(size))
 		return -EINVAL;
 
-	return __kvm_gmem_create(kvm, size, flags);
+	if (flags & GUEST_MEMFD_FLAG_BIND_NODE) {
+		if (args->pad || args->node >= MAX_NUMNODES)
+			return -EINVAL;
+		node = args->node;
+	} else if (args->node || args->pad) {
+		return -EINVAL;
+	}
+
+	return __kvm_gmem_create(kvm, size, flags, node);
 }
 
 int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
-- 
2.53.0-Meta