[PATCH v2 2/2] iommu: Remove iommu_sva_ops::mm_exit()

[Jean-Philippe Brucker]

After binding a device to an mm, device drivers currently need to
register a mm_exit handler. This function is called when the mm exits,
to gracefully stop DMA targeting the address space and flush page faults
to the IOMMU.

This is deemed too complex for the MMU release() notifier, which may be
triggered by any mmput() invocation, from about 120 callsites [1]. The
upcoming SVA module has an example of such complexity: the I/O Page
Fault handler would need to call mmput_async() instead of mmput() after
handling an IOPF, to avoid triggering the release() notifier which would
in turn drain the IOPF queue and lock up.

Another concern is the DMA stop function taking too long, up to several
minutes [2]. For some mmput() callers this may disturb other users. For
example, if the OOM killer picks the mm bound to a device as the victim
and that mm's memory is locked, if the release() takes too long, it
might choose additional innocent victims to kill.

To simplify the MMU release notifier, don't forward the notification to
device drivers. Since they don't stop DMA on mm exit anymore, the PASID
lifetime is extended:

(1) The device driver calls bind(). A PASID is allocated.

  Here any DMA fault is handled by mm, and on error we don't print
  anything to dmesg. Userspace can easily trigger errors by issuing DMA
  on unmapped buffers.

(2) exit_mmap(), for example the process took a SIGKILL. This step
    doesn't happen during normal operations. Remove the pgd from the
    PASID table, since the page tables are about to be freed. Invalidate
    the IOTLBs.

  Here the device may still perform DMA on the address space. Incoming
  transactions are aborted but faults aren't printed out. ATS
  Translation Requests return Successful Translation Completions with
  R=W=0. PRI Page Requests return with Invalid Request.

(3) The device driver stops DMA, possibly following release of a fd, and
    calls unbind(). PASID table is cleared, IOTLB invalidated if
    necessary. The page fault queues are drained, and the PASID is
    freed.

  If DMA for that PASID is still running here, something went seriously
  wrong and errors should be reported.

For now remove iommu_sva_ops entirely. We might need to re-introduce
them at some point, for example to notify device drivers of unhandled
IOPF.

[1] https://lore.kernel.org/linux-iommu/20200306174239.GM31668@ziepe.ca/
[2] https://lore.kernel.org/linux-iommu/4d68da96-0ad5-b412-5987-2f7a6aa796c3@amd.com/

Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
---
v1->v2: tried to clarify the lifetime of the PASID
---
 include/linux/iommu.h | 30 ------------------------------
 drivers/iommu/iommu.c | 11 -----------
 2 files changed, 41 deletions(-)

diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 7ef8b0bda6951..bd330d6343b78 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -53,8 +53,6 @@ struct iommu_fault_event;
 
 typedef int (*iommu_fault_handler_t)(struct iommu_domain *,
 			struct device *, unsigned long, int, void *);
-typedef int (*iommu_mm_exit_handler_t)(struct device *dev, struct iommu_sva *,
-				       void *);
 typedef int (*iommu_dev_fault_handler_t)(struct iommu_fault *, void *);
 
 struct iommu_domain_geometry {
@@ -171,25 +169,6 @@ enum iommu_dev_features {
 
 #define IOMMU_PASID_INVALID	(-1U)
 
-/**
- * struct iommu_sva_ops - device driver callbacks for an SVA context
- *
- * @mm_exit: called when the mm is about to be torn down by exit_mmap. After
- *           @mm_exit returns, the device must not issue any more transaction
- *           with the PASID given as argument.
- *
- *           The @mm_exit handler is allowed to sleep. Be careful about the
- *           locks taken in @mm_exit, because they might lead to deadlocks if
- *           they are also held when dropping references to the mm. Consider the
- *           following call chain:
- *           mutex_lock(A); mmput(mm) -> exit_mm() -> @mm_exit() -> mutex_lock(A)
- *           Using mmput_async() prevents this scenario.
- *
- */
-struct iommu_sva_ops {
-	iommu_mm_exit_handler_t mm_exit;
-};
-
 #ifdef CONFIG_IOMMU_API
 
 /**
@@ -605,7 +584,6 @@ struct iommu_fwspec {
  */
 struct iommu_sva {
 	struct device			*dev;
-	const struct iommu_sva_ops	*ops;
 };
 
 int iommu_fwspec_init(struct device *dev, struct fwnode_handle *iommu_fwnode,
@@ -653,8 +631,6 @@ struct iommu_sva *iommu_sva_bind_device(struct device *dev,
 					struct mm_struct *mm,
 					void *drvdata);
 void iommu_sva_unbind_device(struct iommu_sva *handle);
-int iommu_sva_set_ops(struct iommu_sva *handle,
-		      const struct iommu_sva_ops *ops);
 int iommu_sva_get_pasid(struct iommu_sva *handle);
 
 #else /* CONFIG_IOMMU_API */
@@ -1058,12 +1034,6 @@ static inline void iommu_sva_unbind_device(struct iommu_sva *handle)
 {
 }
 
-static inline int iommu_sva_set_ops(struct iommu_sva *handle,
-				    const struct iommu_sva_ops *ops)
-{
-	return -EINVAL;
-}
-
 static inline int iommu_sva_get_pasid(struct iommu_sva *handle)
 {
 	return IOMMU_PASID_INVALID;
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 2b471419e26c3..dfed12328c032 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2637,17 +2637,6 @@ void iommu_sva_unbind_device(struct iommu_sva *handle)
 }
 EXPORT_SYMBOL_GPL(iommu_sva_unbind_device);
 
-int iommu_sva_set_ops(struct iommu_sva *handle,
-		      const struct iommu_sva_ops *sva_ops)
-{
-	if (handle->ops && handle->ops != sva_ops)
-		return -EEXIST;
-
-	handle->ops = sva_ops;
-	return 0;
-}
-EXPORT_SYMBOL_GPL(iommu_sva_set_ops);
-
 int iommu_sva_get_pasid(struct iommu_sva *handle)
 {
 	const struct iommu_ops *ops = handle->dev->bus->iommu_ops;
-- 
2.26.0