[PATCH v1] KVM: s390: disable migration mode when dirty tracking is disabled

Nico Boehr
2023-01-20T07:54:17Z

Migration mode is a VM attribute which enables tracking of changes in
storage attributes (PGSTE). It assumes dirty tracking is enabled on all
memslots to keep a dirty bitmap of pages with changed storage attributes.

When enabling migration mode, we currently check that dirty tracking is
enabled for all memslots. However, userspace can disable dirty tracking
without disabling migration mode.

Since migration mode is pointless with dirty tracking disabled, disable
migration mode whenever userspace disables dirty tracking on any slot.

Also update the documentation to clarify that dirty tracking must be
enabled when enabling migration mode, which is already enforced by the
code in kvm_s390_vm_start_migration().

To disable migration mode, slots_lock should be held, which is taken
in kvm_set_memory_region() and thus held in
kvm_arch_prepare_memory_region().

Restructure the prepare code a bit so all the sanity checking is done
before disabling migration mode. This ensures migration mode isn't
disabled when some sanity check fails.

Cc: stable@vger.kernel.org
Fixes: 190df4a212a7 ("KVM: s390: CMMA tracking, ESSA emulation, migration mode")
Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
---
Documentation/virt/kvm/devices/vm.rst | 4 +++
arch/s390/kvm/kvm-s390.c | 41 ++++++++++++++++++---------
2 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/Documentation/virt/kvm/devices/vm.rst b/Documentation/virt/kvm/devices/vm.rst
index 60acc39e0e93..147efec626e5 100644
--- a/Documentation/virt/kvm/devices/vm.rst
+++ b/Documentation/virt/kvm/devices/vm.rst
@@ -302,6 +302,10 @@ Allows userspace to start migration mode, needed for PGSTE migration.
Setting this attribute when migration mode is already active will have
no effects.

+Dirty tracking must be enabled on all memslots, else -EINVAL is returned. When
+dirty tracking is disabled on any memslot, migration mode is automatically
+stopped.
+
:Parameters: none
:Returns: -ENOMEM if there is not enough free memory to start migration mode;
-EINVAL if the state of the VM is invalid (e.g. no memory defined);
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index e4890e04b210..4785f002cd93 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -5628,28 +5628,43 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
enum kvm_mr_change change)
{
gpa_t size;
+ int rc;

/* When we are protected, we should not change the memory slots */
if (kvm_s390_pv_get_handle(kvm))
return -EINVAL;

- if (change == KVM_MR_DELETE || change == KVM_MR_FLAGS_ONLY)
- return 0;
+ if (change != KVM_MR_DELETE && change != KVM_MR_FLAGS_ONLY) {
+ /* A few sanity checks. We can have memory slots which have to be
+ * located/ended at a segment boundary (1MB). The memory in userland is
+ * ok to be fragmented into various different vmas. It is okay to mmap()
+ * and munmap() stuff in this slot after doing this call at any time
+ */

- /* A few sanity checks. We can have memory slots which have to be
- located/ended at a segment boundary (1MB). The memory in userland is
- ok to be fragmented into various different vmas. It is okay to mmap()
- and munmap() stuff in this slot after doing this call at any time */
+ if (new->userspace_addr & 0xffffful)
+ return -EINVAL;

- if (new->userspace_addr & 0xffffful)
- return -EINVAL;
+ size = new->npages * PAGE_SIZE;
+ if (size & 0xffffful)
+ return -EINVAL;

- size = new->npages * PAGE_SIZE;
- if (size & 0xffffful)
- return -EINVAL;
+ if ((new->base_gfn * PAGE_SIZE) + size > kvm->arch.mem_limit)
+ return -EINVAL;
+ }

- if ((new->base_gfn * PAGE_SIZE) + size > kvm->arch.mem_limit)
- return -EINVAL;
+ /* Turn off migration mode when userspace disables dirty page logging.
+ * Migration mode expects dirty page logging being enabled to store
+ * its dirty bitmap.
+ */
+ if (kvm->arch.migration_mode) {
+ if ((old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
+ !(new->flags & KVM_MEM_LOG_DIRTY_PAGES)) {
+ rc = kvm_s390_vm_stop_migration(kvm);
+
+ if (rc)
+ pr_warn("Failed to stop migration mode\n");
+ }
+ }

return 0;
}
--
2.39.0