[PATCH v3 3/5] vhost-vdpa: introduce IOTLB_PERSIST backend feature bit

[Si-Wei Liu]

Userspace needs this feature flag to distinguish if vhost-vdpa
iotlb in the kernel can be trusted to persist IOTLB mapping
across vDPA reset. Without it, userspace has no way to tell apart
if it's running on an older kernel, which could silently drop
all iotlb mapping across vDPA reset, especially with broken
parent driver implementation for the .reset driver op. The broken
driver may incorrectly drop all mappings of its own as part of
.reset, which inadvertently ends up with corrupted mapping state
between vhost-vdpa userspace and the kernel. As a workaround, to
make the mapping behaviour predictable across reset, userspace
has to pro-actively remove all mappings before vDPA reset, and
then restore all the mappings afterwards. This workaround is done
unconditionally on top of all parent drivers today, due to the
parent driver implementation issue and no means to differentiate.
This workaround had been utilized in QEMU since day one when the
corresponding vhost-vdpa userspace backend came to the world.

There are 3 cases that backend may claim this feature bit on for:

- parent device that has to work with platform IOMMU
- parent device with on-chip IOMMU that has the expected
  .reset_map support in driver
- parent device with vendor specific IOMMU implementation with
  persistent IOTLB mapping already that has to specifically
  declare this backend feature

The reason why .reset_map is being one of the pre-condition for
persistent iotlb is because without it, vhost-vdpa can't switch
back iotlb to the initial state later on, especially for the
on-chip IOMMU case which starts with identity mapping at device
creation. virtio-vdpa requires on-chip IOMMU to perform 1:1
passthrough translation from PA to IOVA as-is to begin with, and
.reset_map is the only means to turn back iotlb to the identity
mapping mode after vhost-vdpa is gone.

The difference in behavior did not matter as QEMU unmaps all the
memory unregistering the memory listener at vhost_vdpa_dev_start(
started = false), but the backend acknowledging this feature flag
allows QEMU to make sure it is safe to skip this unmap & map in the
case of vhost stop & start cycle.

In that sense, this feature flag is actually a signal for userspace
to know that the driver bug has been solved. Not offering it
indicates that userspace cannot trust the kernel will retain the
maps.

Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
Acked-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
---
 drivers/vhost/vdpa.c             | 15 +++++++++++++++
 include/uapi/linux/vhost_types.h |  2 ++
 2 files changed, 17 insertions(+)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index a3f8160c9807..9202986a7d81 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -438,6 +438,15 @@ static u64 vhost_vdpa_get_backend_features(const struct vhost_vdpa *v)
 		return ops->get_backend_features(vdpa);
 }
 
+static bool vhost_vdpa_has_persistent_map(const struct vhost_vdpa *v)
+{
+	struct vdpa_device *vdpa = v->vdpa;
+	const struct vdpa_config_ops *ops = vdpa->config;
+
+	return (!ops->set_map && !ops->dma_map) || ops->reset_map ||
+	       vhost_vdpa_get_backend_features(v) & BIT_ULL(VHOST_BACKEND_F_IOTLB_PERSIST);
+}
+
 static long vhost_vdpa_set_features(struct vhost_vdpa *v, u64 __user *featurep)
 {
 	struct vdpa_device *vdpa = v->vdpa;
@@ -725,6 +734,7 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 			return -EFAULT;
 		if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
 				 BIT_ULL(VHOST_BACKEND_F_DESC_ASID) |
+				 BIT_ULL(VHOST_BACKEND_F_IOTLB_PERSIST) |
 				 BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
 				 BIT_ULL(VHOST_BACKEND_F_RESUME) |
 				 BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK)))
@@ -741,6 +751,9 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 		if ((features & BIT_ULL(VHOST_BACKEND_F_DESC_ASID)) &&
 		     !vhost_vdpa_has_desc_group(v))
 			return -EOPNOTSUPP;
+		if ((features & BIT_ULL(VHOST_BACKEND_F_IOTLB_PERSIST)) &&
+		     !vhost_vdpa_has_persistent_map(v))
+			return -EOPNOTSUPP;
 		vhost_set_backend_features(&v->vdev, features);
 		return 0;
 	}
@@ -796,6 +809,8 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 			features |= BIT_ULL(VHOST_BACKEND_F_RESUME);
 		if (vhost_vdpa_has_desc_group(v))
 			features |= BIT_ULL(VHOST_BACKEND_F_DESC_ASID);
+		if (vhost_vdpa_has_persistent_map(v))
+			features |= BIT_ULL(VHOST_BACKEND_F_IOTLB_PERSIST);
 		features |= vhost_vdpa_get_backend_features(v);
 		if (copy_to_user(featurep, &features, sizeof(features)))
 			r = -EFAULT;
diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
index 18ad6ae7ab5c..d7656908f730 100644
--- a/include/uapi/linux/vhost_types.h
+++ b/include/uapi/linux/vhost_types.h
@@ -190,5 +190,7 @@ struct vhost_vdpa_iova_range {
  * buffers may reside. Requires VHOST_BACKEND_F_IOTLB_ASID.
  */
 #define VHOST_BACKEND_F_DESC_ASID    0x7
+/* IOTLB don't flush memory mapping across device reset */
+#define VHOST_BACKEND_F_IOTLB_PERSIST  0x8
 
 #endif
-- 
2.39.3