Re: [PATCH v4 03/26] iommu: Add a page fault handler

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Hi,

On 2020/2/25 2:23, Jean-Philippe Brucker wrote:
From: Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx>

Some systems allow devices to handle I/O Page Faults in the core mm. For
example systems implementing the PCI PRI extension or Arm SMMU stall
model. Infrastructure for reporting these recoverable page faults was
recently added to the IOMMU core. Add a page fault handler for host SVA.

IOMMU driver can now instantiate several fault workqueues and link them to
IOPF-capable devices. Drivers can choose between a single global
workqueue, one per IOMMU device, one per low-level fault queue, one per
domain, etc.

When it receives a fault event, supposedly in an IRQ handler, the IOMMU
driver reports the fault using iommu_report_device_fault(), which calls
the registered handler. The page fault handler then calls the mm fault
handler, and reports either success or failure with iommu_page_response().
When the handler succeeded, the IOMMU retries the access.

The iopf_param pointer could be embedded into iommu_fault_param. But
putting iopf_param into the iommu_param structure allows us not to care
about ordering between calls to iopf_queue_add_device() and
iommu_register_device_fault_handler().

Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
---
  drivers/iommu/Kconfig      |   4 +
  drivers/iommu/Makefile     |   1 +
  drivers/iommu/io-pgfault.c | 451 +++++++++++++++++++++++++++++++++++++
  include/linux/iommu.h      |  59 +++++
  4 files changed, 515 insertions(+)
  create mode 100644 drivers/iommu/io-pgfault.c

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index acca20e2da2f..e4a42e1708b4 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -109,6 +109,10 @@ config IOMMU_SVA
  	select IOMMU_API
  	select MMU_NOTIFIER
+config IOMMU_PAGE_FAULT
+	bool
+	select IOMMU_API
+
  config FSL_PAMU
  	bool "Freescale IOMMU support"
  	depends on PCI
diff --git a/drivers/iommu/Makefile b/drivers/iommu/Makefile
index 40c800dd4e3e..bf5cb4ee8409 100644
--- a/drivers/iommu/Makefile
+++ b/drivers/iommu/Makefile
@@ -4,6 +4,7 @@ obj-$(CONFIG_IOMMU_API) += iommu-traces.o
  obj-$(CONFIG_IOMMU_API) += iommu-sysfs.o
  obj-$(CONFIG_IOMMU_DEBUGFS) += iommu-debugfs.o
  obj-$(CONFIG_IOMMU_DMA) += dma-iommu.o
+obj-$(CONFIG_IOMMU_PAGE_FAULT) += io-pgfault.o
  obj-$(CONFIG_IOMMU_IO_PGTABLE) += io-pgtable.o
  obj-$(CONFIG_IOMMU_IO_PGTABLE_ARMV7S) += io-pgtable-arm-v7s.o
  obj-$(CONFIG_IOMMU_IO_PGTABLE_LPAE) += io-pgtable-arm.o
diff --git a/drivers/iommu/io-pgfault.c b/drivers/iommu/io-pgfault.c
new file mode 100644
index 000000000000..76e153c59fe3
--- /dev/null
+++ b/drivers/iommu/io-pgfault.c
@@ -0,0 +1,451 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Handle device page faults
+ *
+ * Copyright (C) 2018 ARM Ltd.
+ */
+
+#include <linux/iommu.h>
+#include <linux/list.h>
+#include <linux/slab.h>
+#include <linux/workqueue.h>
+
+/**
+ * struct iopf_queue - IO Page Fault queue
+ * @wq: the fault workqueue
+ * @flush: low-level flush callback
+ * @flush_arg: flush() argument
+ * @devices: devices attached to this queue
+ * @lock: protects the device list
+ */
+struct iopf_queue {
+	struct workqueue_struct		*wq;
+	iopf_queue_flush_t		flush;
+	void				*flush_arg;
+	struct list_head		devices;
+	struct mutex			lock;
+};
+
+/**
+ * struct iopf_device_param - IO Page Fault data attached to a device
+ * @dev: the device that owns this param
+ * @queue: IOPF queue
+ * @queue_list: index into queue->devices
+ * @partial: faults that are part of a Page Request Group for which the last
+ *           request hasn't been submitted yet.
+ * @busy: the param is being used
+ * @wq_head: signal a change to @busy
+ */
+struct iopf_device_param {
+	struct device			*dev;
+	struct iopf_queue		*queue;
+	struct list_head		queue_list;
+	struct list_head		partial;
+	bool				busy;
+	wait_queue_head_t		wq_head;
+};
+
+struct iopf_fault {
+	struct iommu_fault		fault;
+	struct list_head		head;
+};
+
+struct iopf_group {
+	struct iopf_fault		last_fault;
+	struct list_head		faults;
+	struct work_struct		work;
+	struct device			*dev;
+};
+
[...]
+
+/**
+ * iopf_queue_alloc - Allocate and initialize a fault queue
+ * @name: a unique string identifying the queue (for workqueue)
+ * @flush: a callback that flushes the low-level queue
+ * @cookie: driver-private data passed to the flush callback
+ *
+ * The callback is called before the workqueue is flushed. The IOMMU driver must
+ * commit all faults that are pending in its low-level queues at the time of the
+ * call, into the IOPF queue (with iommu_report_device_fault). The callback
+ * takes a device pointer as argument, hinting what endpoint is causing the
+ * flush. When the device is NULL, all faults should be committed.
+ *
+ * Return: the queue on success and NULL on error.
+ */
+struct iopf_queue *
+iopf_queue_alloc(const char *name, iopf_queue_flush_t flush, void *cookie)
+{
+	struct iopf_queue *queue;
+
+	queue = kzalloc(sizeof(*queue), GFP_KERNEL);
+	if (!queue)
+		return NULL;
+
+	/*
+	 * The WQ is unordered because the low-level handler enqueues faults by
+	 * group. PRI requests within a group have to be ordered, but once
+	 * that's dealt with, the high-level function can handle groups out of
+	 * order.
+	 */
+	queue->wq = alloc_workqueue("iopf_queue/%s", WQ_UNBOUND, 0, name);
Should this workqueue use 'WQ_HIGHPRI | WQ_UNBOUND' or some flags like this to decrease the unexpected latency of I/O PageFault here? Or maybe, workqueue will show an uncontrolled latency, even in a busy system.
Cheers,
Zaibo

.
+	if (!queue->wq) {
+		kfree(queue);
+		return NULL;
+	}
+
+	queue->flush = flush;
+	queue->flush_arg = cookie;
+	INIT_LIST_HEAD(&queue->devices);
+	mutex_init(&queue->lock);
+
+	return queue;
+}
+EXPORT_SYMBOL_GPL(iopf_queue_alloc);
+
+/**
+ * iopf_queue_free - Free IOPF queue
+ * @queue: queue to free
+ *
+ * Counterpart to iopf_queue_alloc(). The driver must not be queuing faults or
+ * adding/removing devices on this queue anymore.
+ */
+void iopf_queue_free(struct iopf_queue *queue)
+{
+	struct iopf_device_param *iopf_param, *next;
+
+	if (!queue)
+		return;
+
+	list_for_each_entry_safe(iopf_param, next, &queue->devices, queue_list)
+		iopf_queue_remove_device(queue, iopf_param->dev);
+
+	destroy_workqueue(queue->wq);
+	kfree(queue);
+}
+EXPORT_SYMBOL_GPL(iopf_queue_free);
[...]




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