[RFC PATCH v4 14/25] vfio/pci: Add helpers to check and serialize BAR state changes

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

Add vfio_pci_begin_bar_state_change() to check whether a BAR state
change is allowed before taking vdev->memory_lock for write and removing
existing BAR mappings with vfio_pci_zap_bars(). Return -EBUSY if the
check fails, before removing mappings or changing hardware.

Keep memory_lock held on success until the caller finishes the hardware
operation and calls vfio_pci_end_bar_state_change().

vfio_pci_bar_state_change_allowed() initially returns true. Later
patches use these helpers for command-register writes, reset and power
changes, then reject those operations while the device supplies
guest_memfd backing.

Cc: Alex Williamson <alex@shazbot.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 drivers/vfio/pci/vfio_pci_core.c | 15 +++++++++++++++
 drivers/vfio/pci/vfio_pci_priv.h | 10 ++++++++++
 2 files changed, 25 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index fcbb2da860e2..99778cf0cc3c 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1709,6 +1709,21 @@ void vfio_pci_zap_and_down_write_memory_lock(struct vfio_pci_core_device *vdev)
 	vfio_pci_zap_bars(vdev);
 }
 
+int vfio_pci_begin_bar_state_change(struct vfio_pci_core_device *vdev)
+{
+	if (!vfio_pci_bar_state_change_allowed(vdev))
+		return -EBUSY;
+
+	down_write(&vdev->memory_lock);
+	vfio_pci_zap_bars(vdev);
+	return 0;
+}
+
+void vfio_pci_end_bar_state_change(struct vfio_pci_core_device *vdev)
+{
+	up_write(&vdev->memory_lock);
+}
+
 u16 vfio_pci_memory_lock_and_enable(struct vfio_pci_core_device *vdev)
 {
 	u16 cmd;
diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
index 4e7162234a2e..0af93f6d9a8a 100644
--- a/drivers/vfio/pci/vfio_pci_priv.h
+++ b/drivers/vfio/pci/vfio_pci_priv.h
@@ -11,6 +11,13 @@
 /* Cap maximum number of ioeventfds per device (arbitrary) */
 #define VFIO_PCI_IOEVENTFD_MAX		1000
 
+/* Later patches reject BAR state changes while guest_memfd uses the device. */
+static inline bool
+vfio_pci_bar_state_change_allowed(struct vfio_pci_core_device *vdev)
+{
+	return true;
+}
+
 struct vfio_pci_ioeventfd {
 	struct list_head	next;
 	struct vfio_pci_core_device	*vdev;
@@ -69,6 +76,9 @@ int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev,
 			     pci_power_t state);
 
 void vfio_pci_zap_and_down_write_memory_lock(struct vfio_pci_core_device *vdev);
+/* Success retains memory_lock through the hardware operation. */
+int vfio_pci_begin_bar_state_change(struct vfio_pci_core_device *vdev);
+void vfio_pci_end_bar_state_change(struct vfio_pci_core_device *vdev);
 u16 vfio_pci_memory_lock_and_enable(struct vfio_pci_core_device *vdev);
 void vfio_pci_memory_unlock_and_restore(struct vfio_pci_core_device *vdev,
 					u16 cmd);
-- 
2.43.0