[RFC PATCH v4 22/25] vfio/pci: Require D0 before providing guest_memfd BAR backing

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

Guest_memfd needs the device in D0 while its BARs supply backing memory.
Lower-power states can limit device operation or make BAR accesses
unavailable; D3cold removes power from the device entirely.

Add vfio_pci_core_pm_pin_d0() to resume the device and retain a
runtime-PM reference. Check current_state under memory_lock and return
EBUSY unless the device is in D0. An explicit PMCSR write can leave the
device in a lower power state even when runtime PM considers it active.

The retained reference prevents runtime suspend. The caller must also
prevent explicit power-state changes and drop the reference with
pm_runtime_put() when finished. The following provider patch holds it
until provider release.

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 | 28 ++++++++++++++++++++++++++++
 drivers/vfio/pci/vfio_pci_priv.h |  2 ++
 2 files changed, 30 insertions(+)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index b8e574194f07..1c606da81879 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -370,6 +370,34 @@ int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev, pci_power_t stat
 	return ret;
 }
 
+/**
+ * vfio_pci_core_pm_pin_d0() - resume a device and retain runtime PM
+ * @vdev: device whose runtime-PM reference is acquired
+ *
+ * Require D0 after resuming: explicit PMCSR writes can leave a
+ * runtime-active function in a lower power state. The caller must exclude
+ * explicit power-state changes while relying on this reference.
+ *
+ * Return: 0 with a reference to release using pm_runtime_put(), or a negative
+ * error with no reference held.
+ */
+int vfio_pci_core_pm_pin_d0(struct vfio_pci_core_device *vdev)
+{
+	int ret;
+
+	ret = pm_runtime_resume_and_get(&vdev->pdev->dev);
+	if (ret < 0)
+		return ret;
+
+	down_read(&vdev->memory_lock);
+	ret = vdev->pdev->current_state == PCI_D0 ? 0 : -EBUSY;
+	up_read(&vdev->memory_lock);
+	if (ret)
+		pm_runtime_put(&vdev->pdev->dev);
+
+	return ret;
+}
+
 static int vfio_pci_runtime_pm_entry(struct vfio_pci_core_device *vdev,
 				     struct eventfd_ctx *efdctx)
 {
diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
index a2501759e805..43d74c46e841 100644
--- a/drivers/vfio/pci/vfio_pci_priv.h
+++ b/drivers/vfio/pci/vfio_pci_priv.h
@@ -78,6 +78,8 @@ void vfio_pci_uninit_perm_bits(void);
 int vfio_config_init(struct vfio_pci_core_device *vdev);
 void vfio_config_free(struct vfio_pci_core_device *vdev);
 
+int vfio_pci_core_pm_pin_d0(struct vfio_pci_core_device *vdev);
+
 int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev,
 			     pci_power_t state);
 
-- 
2.43.0