[PATCH v3 4/9] KVM: guest_memfd: add GUEST_MEMFD_FLAG_MIGRATABLE
From: Shivank Garg
Date: Wed Aug 05 2026 - 02:46:22 EST
guest_memfd folios are currently always marked unmovable, which
prevents the MM from invoking ->migrate_folio callback in scenarios
like compaction, offlining, etc. This is unavoidable for confidential VMs
(SEV-SNP, TDX), since memory is encrypted and copying it needs firmware
assistance. However, for non-confidential VMs (like firecracker), we
can still migrate the folios.
Add kvm_arch_supports_gmem_migration() so an architecture can control
whether migration is supported. Add GUEST_MEMFD_FLAG_MIGRATABLE so
userspace can opt-in to folio migration.
For opted-in guest_memfds, use GFP_HIGHUSER_MOVABLE so their folios can
be allocated from ZONE_MOVABLE/CMA and grouped by mobility, otherwise
keep the existing unmovable behavior.
Remove redundant WARN_ON_ONCE for checking unevictable mapping.
Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
Documentation/virt/kvm/api.rst | 3 +++
arch/x86/kvm/x86.c | 9 +++++++++
include/linux/kvm_host.h | 4 ++++
include/uapi/linux/kvm.h | 1 +
tools/include/uapi/linux/kvm.h | 1 +
virt/kvm/guest_memfd.c | 18 ++++++++++++++----
6 files changed, 32 insertions(+), 4 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index e3003a241d5b..976921527ef9 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6461,6 +6461,9 @@ specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags:
without INIT_SHARED will be marked private).
Shared memory can be faulted into host userspace
page tables. Private memory cannot.
+ GUEST_MEMFD_FLAG_MIGRATABLE Allow MM to migrate guest_memfd folios.
+ Availability is architecture sepecific. Without
+ this flag, folios remain unmovable.
============================ ================================================
When the KVM MMU performs a PFN lookup to service a guest fault and the backing
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 47cb9eba113b..7087b0b8f973 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -14130,6 +14130,15 @@ bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
return !kvm_arch_has_private_mem(kvm);
}
+/*
+ * Migration of guest_memfd with private memory is not supported yet
+ * as this may require architecture-specific handling.
+ */
+bool kvm_arch_supports_gmem_migration(struct kvm *kvm)
+{
+ return !kvm_arch_has_private_mem(kvm);
+}
+
#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order)
{
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ab8cfaec82d3..7b2f15b21970 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -731,6 +731,7 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
#ifdef CONFIG_KVM_GUEST_MEMFD
bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm);
+bool kvm_arch_supports_gmem_migration(struct kvm *kvm);
static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
{
@@ -738,6 +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_MIGRATION) &&
+ (!kvm || kvm_arch_supports_gmem_migration(kvm)))
+ flags |= GUEST_MEMFD_FLAG_MIGRATABLE;
return flags;
}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 419011097fa8..e1615a62ed78 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1654,6 +1654,7 @@ 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_MIGRATABLE (1ULL << 2)
struct kvm_create_guest_memfd {
__u64 size;
diff --git a/tools/include/uapi/linux/kvm.h b/tools/include/uapi/linux/kvm.h
index 419011097fa8..e1615a62ed78 100644
--- a/tools/include/uapi/linux/kvm.h
+++ b/tools/include/uapi/linux/kvm.h
@@ -1654,6 +1654,7 @@ 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_MIGRATABLE (1ULL << 2)
struct kvm_create_guest_memfd {
__u64 size;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 583341d593c9..a2b7c2f1c49f 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -589,6 +589,11 @@ bool __weak kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
return true;
}
+bool __weak kvm_arch_supports_gmem_migration(struct kvm *kvm)
+{
+ return false;
+}
+
static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
{
static const char *name = "[kvm-gmem]";
@@ -623,11 +628,16 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
inode->i_mapping->a_ops = &kvm_gmem_aops;
inode->i_mode |= S_IFREG;
inode->i_size = size;
- mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
mapping_set_inaccessible(inode->i_mapping);
- mapping_set_unmovable(inode->i_mapping);
- /* Unmovable mappings are supposed to be marked unevictable as well. */
- WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
+ /* guest_memfd mappings should be marked unevictable. */
+ mapping_set_unevictable(inode->i_mapping);
+
+ if (flags & GUEST_MEMFD_FLAG_MIGRATABLE) {
+ mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER_MOVABLE);
+ } else {
+ mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
+ mapping_set_unmovable(inode->i_mapping);
+ }
GMEM_I(inode)->flags = flags;
--
2.43.0