[PATCH,RFC] iommu/amd: Recover from event log overflow

[Lennert Buytenhek <buytenh@wantstofly.org>]

The AMD IOMMU logs I/O page faults and such to a ring buffer in
system memory, and this ring buffer can overflow.  The AMD IOMMU
spec has the following to say about the interrupt status bit that
signals this overflow condition:

	EventOverflow: Event log overflow. RW1C. Reset 0b. 1 = IOMMU
	event log overflow has occurred. This bit is set when a new
	event is to be written to the event log and there is no usable
	entry in the event log, causing the new event information to
	be discarded. An interrupt is generated when EventOverflow = 1b
	and MMIO Offset 0018h[EventIntEn] = 1b. No new event log
	entries are written while this bit is set. Software Note: To
	resume logging, clear EventOverflow (W1C), and write a 1 to
	MMIO Offset 0018h[EventLogEn].

The AMD IOMMU driver doesn't currently implement this recovery
sequence, meaning that if a ring buffer overflow occurs, logging of
EVT/PPR/GA events will cease entirely.

This patch implements the spec-mandated reset sequence, with the minor
tweak that the hardware seems to want to have a 0 written to MMIO Offset
0018h[EventLogEn] first, before writing an 1 into this field, or the
IOMMU won't actually resume logging events.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
---
N.B.: I have only tested this change against an older in-house custom
kernel version.  (Where it appears to survive stress testing.)  This
version of the patch was only compile-tested.

This patch makes iommu_feature_{disable,enable}() be called at non-init
time (via amd_iommu_restart_event_logging()), and those functions perform
unguarded read-modify-write sequences on the MMIO_CONTROL_OFFSET register,
and I haven't yet verified whether that is safe to do.

Reproducing this issue is fairly easy if you can easily cause I/O page
faults -- either stick an msleep() in the amd_iommu_int_thread() loop,
or configure a slow serial console (the latter being how this was
initially found).

 drivers/iommu/amd/amd_iommu.h       |  1 +
 drivers/iommu/amd/amd_iommu_types.h |  1 +
 drivers/iommu/amd/init.c            | 10 ++++++++++
 drivers/iommu/amd/iommu.c           | 10 ++++++++--
 4 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/amd/amd_iommu.h b/drivers/iommu/amd/amd_iommu.h
index 416815a525d6..bb95edf74415 100644
--- a/drivers/iommu/amd/amd_iommu.h
+++ b/drivers/iommu/amd/amd_iommu.h
@@ -14,6 +14,7 @@
 extern irqreturn_t amd_iommu_int_thread(int irq, void *data);
 extern irqreturn_t amd_iommu_int_handler(int irq, void *data);
 extern void amd_iommu_apply_erratum_63(u16 devid);
+extern void amd_iommu_restart_event_logging(struct amd_iommu *iommu);
 extern void amd_iommu_reset_cmd_buffer(struct amd_iommu *iommu);
 extern int amd_iommu_init_devices(void);
 extern void amd_iommu_uninit_devices(void);
diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
index 94c1a7a9876d..34b90f8b2056 100644
--- a/drivers/iommu/amd/amd_iommu_types.h
+++ b/drivers/iommu/amd/amd_iommu_types.h
@@ -110,6 +110,7 @@
 #define PASID_MASK		0x0000ffff
 
 /* MMIO status bits */
+#define MMIO_STATUS_EVT_OVERFLOW_INT_MASK	(1 << 0)
 #define MMIO_STATUS_EVT_INT_MASK	(1 << 1)
 #define MMIO_STATUS_COM_WAIT_INT_MASK	(1 << 2)
 #define MMIO_STATUS_PPR_INT_MASK	(1 << 6)
diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 46280e6e1535..44d1536474ed 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -639,6 +639,16 @@ static int __init alloc_command_buffer(struct amd_iommu *iommu)
 	return iommu->cmd_buf ? 0 : -ENOMEM;
 }
 
+/*
+ * This function restarts event logging in case the IOMMU experienced
+ * an event log buffer overflow.
+ */
+void amd_iommu_restart_event_logging(struct amd_iommu *iommu)
+{
+	iommu_feature_disable(iommu, CONTROL_EVT_LOG_EN);
+	iommu_feature_enable(iommu, CONTROL_EVT_LOG_EN);
+}
+
 /*
  * This function resets the command buffer if the IOMMU stopped fetching
  * commands from it.
diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 811a49a95d04..b04f0cd5ffc2 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -736,7 +736,8 @@ amd_iommu_set_pci_msi_domain(struct device *dev, struct amd_iommu *iommu) { }
 #endif /* !CONFIG_IRQ_REMAP */
 
 #define AMD_IOMMU_INT_MASK	\
-	(MMIO_STATUS_EVT_INT_MASK | \
+	(MMIO_STATUS_EVT_OVERFLOW_INT_MASK | \
+	 MMIO_STATUS_EVT_INT_MASK | \
 	 MMIO_STATUS_PPR_INT_MASK | \
 	 MMIO_STATUS_GALOG_INT_MASK)
 
@@ -746,7 +747,7 @@ irqreturn_t amd_iommu_int_thread(int irq, void *data)
 	u32 status = readl(iommu->mmio_base + MMIO_STATUS_OFFSET);
 
 	while (status & AMD_IOMMU_INT_MASK) {
-		/* Enable EVT and PPR and GA interrupts again */
+		/* Enable interrupt sources again */
 		writel(AMD_IOMMU_INT_MASK,
 			iommu->mmio_base + MMIO_STATUS_OFFSET);
 
@@ -767,6 +768,11 @@ irqreturn_t amd_iommu_int_thread(int irq, void *data)
 		}
 #endif
 
+		if (status & MMIO_STATUS_EVT_OVERFLOW_INT_MASK) {
+			pr_info("AMD-Vi: IOMMU event log overflow\n");
+			amd_iommu_restart_event_logging(iommu);
+		}
+
 		/*
 		 * Hardware bug: ERBT1312
 		 * When re-enabling interrupt (by writing 1
-- 
2.31.1
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