Re: [PATCH] iommu/arm-smmu-v3: Add tracepoint for EVTQ events

[Robin Murphy <robin.murphy@arm.com>]

On 24/06/2026 9:43 am, chenjun (AM) wrote:
> 在 2026/6/23 23:31, Robin Murphy 写道:
>> On 13/06/2026 2:00 pm, Chen Jun wrote:
>>> Events reported by the SMMU can severely impact accelerator
>>> performance. Currently, only events that the SMMU fails to handle are
>>> printed to the kernel log, leaving most events invisible to users.
>>> To analyze and optimize accelerator performance, complete visibility
>>> into all SMMU-reported events is required.
>>
>> What events, exactly? AFAICS the only events we should expect to handle
>> "invisibly", without being some unexpected error condition worth
>> screaming about, would be stalls for SVA page faults, and if SVA isn't
>> generically accounting page faults itself then I would imagine it
>> probably should.
>>
>> Thanks,
>> Robin.
>>
> 
> AF and WP faults are common occurrences. and they can significantly
> impact SMMU performance. If we can determine exactly at which address
> and what type of page fault occurred, it would help us avoid SVA page
> fault events through other means. Also, I don't see any separate
> accounting for page fault events in the SVA flow.

Right, and that's what I'm getting at. By nature, handling IOMMU page 
faults is always going to be less efficient than a CPU handling its own 
fault synchronously; that is not unique to Arm SMMU. The methods for 
pre-faulting pages from the CPU side to minimise avoidable IOMMU faults 
are not unique to Arm SMMU either. And if you care about this, then it's 
highly likely that other SVA users across all architectures will care 
about it too. Thus it seems like a pretty clear argument for having 
something nice like /proc/vmstat for SVA processes to give users 
sufficiently visible accounting of IOMMU faults to decide whether it's 
worth tuning their application.

Sure that's not going to give insight into the details of individual 
faults, but I'd imagine that the majority of cases won't actually need 
that anyway, as they're unlikely to be deeply application-specific and 
requiring analysis at the per-page level - I'd expect most of the value 
to be in merely being able to confirm that there was a high rate of 
faults to begin with, and e.g. adding MAP_POPULATE to some obvious mmap 
calls reduced that rate by X% accounting for a Y% performance 
improvement. But if someone does want the details then even then, a 
common tracepoint in the SVA IOPF path would make a lot more sense than 
an SMMU-specific thing which won't even account for all of SMMU (since 
PRI-based SVA is finally being revived now as well.)

Thanks,
Robin.

> 
> Thanks
> Chen Jun
> 
>>> Add a tracepoint in the EVTQ interrupt handler to capture every
>>> event record reported by the SMMU. This allows users to collect all
>>> event information via ftrace/perf for further analysis, complementing
>>> the existing event decoder and error dump which only cover a subset
>>> of events.
>>>
>>> Signed-off-by: Chen Jun <chenjun102@huawei.com>
>>> ---
>>>     drivers/iommu/arm/arm-smmu-v3/Makefile      |  2 +-
>>>     drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c |  3 ++
>>>     drivers/iommu/arm/arm-smmu-v3/trace.c       |  9 ++++
>>>     drivers/iommu/arm/arm-smmu-v3/trace.h       | 53 +++++++++++++++++++++
>>>     4 files changed, 66 insertions(+), 1 deletion(-)
>>>     create mode 100644 drivers/iommu/arm/arm-smmu-v3/trace.c
>>>     create mode 100644 drivers/iommu/arm/arm-smmu-v3/trace.h
>>>
>>> diff --git a/drivers/iommu/arm/arm-smmu-v3/Makefile b/drivers/iommu/arm/arm-smmu-v3/Makefile
>>> index 493a659cc66b..63a8d71bfc93 100644
>>> --- a/drivers/iommu/arm/arm-smmu-v3/Makefile
>>> +++ b/drivers/iommu/arm/arm-smmu-v3/Makefile
>>> @@ -1,6 +1,6 @@
>>>     # SPDX-License-Identifier: GPL-2.0
>>>     obj-$(CONFIG_ARM_SMMU_V3) += arm_smmu_v3.o
>>> -arm_smmu_v3-y := arm-smmu-v3.o
>>> +arm_smmu_v3-y := arm-smmu-v3.o trace.o
>>>     arm_smmu_v3-$(CONFIG_ARM_SMMU_V3_IOMMUFD) += arm-smmu-v3-iommufd.o
>>>     arm_smmu_v3-$(CONFIG_ARM_SMMU_V3_SVA) += arm-smmu-v3-sva.o
>>>     arm_smmu_v3-$(CONFIG_TEGRA241_CMDQV) += tegra241-cmdqv.o
>>> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>> index e8d7dbe495f0..85e6c25b73ed 100644
>>> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>>> @@ -34,6 +34,8 @@
>>>     #include "arm-smmu-v3.h"
>>>     #include "../../dma-iommu.h"
>>>     
>>> +#include "trace.h"
>>> +
>>>     static bool disable_msipolling;
>>>     module_param(disable_msipolling, bool, 0444);
>>>     MODULE_PARM_DESC(disable_msipolling,
>>> @@ -2271,6 +2273,7 @@ static irqreturn_t arm_smmu_evtq_thread(int irq, void *dev)
>>>     
>>>     	do {
>>>     		while (!queue_remove_raw(q, evt)) {
>>> +			trace_smmu_evtq_event(smmu, evt);
>>>     			arm_smmu_decode_event(smmu, evt, &event);
>>>     			if (arm_smmu_handle_event(smmu, evt, &event))
>>>     				arm_smmu_dump_event(smmu, evt, &event, &rs);
>>> diff --git a/drivers/iommu/arm/arm-smmu-v3/trace.c b/drivers/iommu/arm/arm-smmu-v3/trace.c
>>> new file mode 100644
>>> index 000000000000..77378698b1a3
>>> --- /dev/null
>>> +++ b/drivers/iommu/arm/arm-smmu-v3/trace.c
>>> @@ -0,0 +1,9 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +/*
>>> + * ARM SMMUv3 trace support
>>> + *
>>> + * Copyright (c) 2026 OpenCloudOS / openEuler
>>> + */
>>> +
>>> +#define CREATE_TRACE_POINTS
>>> +#include "trace.h"
>>> diff --git a/drivers/iommu/arm/arm-smmu-v3/trace.h b/drivers/iommu/arm/arm-smmu-v3/trace.h
>>> new file mode 100644
>>> index 000000000000..7cec8d41745e
>>> --- /dev/null
>>> +++ b/drivers/iommu/arm/arm-smmu-v3/trace.h
>>> @@ -0,0 +1,53 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +/*
>>> + * ARM SMMUv3 trace support
>>> + *
>>> + * Copyright (c) 2026 OpenCloudOS / openEuler
>>> + */
>>> +
>>> +#undef TRACE_SYSTEM
>>> +#define TRACE_SYSTEM arm_smmu_v3
>>> +
>>> +#if !defined(_TRACE_ARM_SMMU_V3_H) || defined(TRACE_HEADER_MULTI_READ)
>>> +#define _TRACE_ARM_SMMU_V3_H
>>> +
>>> +#include <linux/tracepoint.h>
>>> +
>>> +#include "arm-smmu-v3.h"
>>> +
>>> +TRACE_EVENT(smmu_evtq_event,
>>> +
>>> +	TP_PROTO(struct arm_smmu_device *smmu, u64 *evt),
>>> +
>>> +	TP_ARGS(smmu, evt),
>>> +
>>> +	TP_STRUCT__entry(
>>> +		__string(iommu, dev_name(smmu->dev))
>>> +		__field(u64, evt0)
>>> +		__field(u64, evt1)
>>> +		__field(u64, evt2)
>>> +		__field(u64, evt3)
>>> +	),
>>> +
>>> +	TP_fast_assign(
>>> +		__assign_str(iommu);
>>> +		__entry->evt0 = evt[0];
>>> +		__entry->evt1 = evt[1];
>>> +		__entry->evt2 = evt[2];
>>> +		__entry->evt3 = evt[3];
>>> +	),
>>> +
>>> +	TP_printk("%s evt: 0x%016llx 0x%016llx 0x%016llx 0x%016llx",
>>> +		__get_str(iommu),
>>> +		__entry->evt0, __entry->evt1,
>>> +		__entry->evt2, __entry->evt3)
>>> +);
>>> +
>>> +#endif /* _TRACE_ARM_SMMU_V3_H */
>>> +
>>> +/* This part must be outside protection */
>>> +#undef TRACE_INCLUDE_PATH
>>> +#undef TRACE_INCLUDE_FILE
>>> +#define TRACE_INCLUDE_PATH ../../drivers/iommu/arm/arm-smmu-v3/
>>> +#define TRACE_INCLUDE_FILE trace
>>> +#include <trace/define_trace.h>
>>
>>
>>
>