Message-ID: <b0dadc40-638e-4f14-8a5c-70c306bc4650@arm.com>
Date: Fri, 17 Jan 2025 12:45:10 +0000
From: Robin Murphy <robin.murphy@....com>
To: Zong Li <zong.li@...ive.com>
Cc: joro@...tes.org, will@...nel.org, tjeznach@...osinc.com,
 paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
 luxu.kernel@...edance.com, linux-kernel@...r.kernel.org,
 iommu@...ts.linux.dev, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 2/2] iommu/riscv: support HPM and interrupt handling

On 2025-01-17 2:46 am, Zong Li wrote:
> On Thu, Jan 16, 2025 at 5:56 AM Robin Murphy <robin.murphy@....com> wrote:
>>
>> On 2025-01-15 3:03 am, Zong Li wrote:
>>> Initialize the PMU and uninitialize it when driver is removed.
>>> Interrupt handling is also implemented, and the handler needs
>>> to be a primary handler instead of a threaded function because
>>> pt_regs is empty when threading the IRQ. However, pt_regs is
>>> required by perf_event_overflow.
>>>
>>> Signed-off-by: Zong Li <zong.li@...ive.com>
>>> Tested-by: Xu Lu <luxu.kernel@...edance.com>
>>> ---
>>>    drivers/iommu/riscv/iommu.c | 65 +++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 65 insertions(+)
>>>
>>> diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
>>> index 8a05def774bd..20ae90471484 100644
>>> --- a/drivers/iommu/riscv/iommu.c
>>> +++ b/drivers/iommu/riscv/iommu.c
>>> @@ -552,6 +552,62 @@ static irqreturn_t riscv_iommu_fltq_process(int irq, void *data)
>>>        return IRQ_HANDLED;
>>>    }
>>>
>>> +/*
>>> + * IOMMU Hardware performance monitor
>>> + */
>>> +
>>> +/* HPM interrupt primary handler */
>>> +static irqreturn_t riscv_iommu_hpm_irq_handler(int irq, void *dev_id)
>>> +{
>>> +     struct riscv_iommu_device *iommu = (struct riscv_iommu_device *)dev_id;
>>> +
>>> +     /* Clear performance monitoring interrupt pending */
>>> +     riscv_iommu_writel(iommu, RISCV_IOMMU_REG_IPSR, RISCV_IOMMU_IPSR_PMIP);
>>> +
>>> +     /* Process pmu irq */
>>> +     riscv_iommu_pmu_handle_irq(&iommu->pmu);
>>> +
>>> +     return IRQ_HANDLED;
>>> +}
>>> +
>>> +/* HPM initialization */
>>> +static int riscv_iommu_hpm_enable(struct riscv_iommu_device *iommu)
>>> +{
>>> +     int rc;
>>> +
>>> +     if (!(iommu->caps & RISCV_IOMMU_CAPABILITIES_HPM))
>>> +             return 0;
>>> +
>>> +     /*
>>> +      * pt_regs is empty when threading the IRQ, but pt_regs is necessary
>>> +      * by perf_event_overflow. Use primary handler instead of thread
>>> +      * function for PM IRQ.
>>> +      *
>>> +      * Set the IRQF_ONESHOT flag because this IRQ might be shared with
>>> +      * other threaded IRQs by other queues.
>>> +      */
>>> +     rc = devm_request_irq(iommu->dev,
>>> +                           iommu->irqs[riscv_iommu_queue_vec(iommu, RISCV_IOMMU_IPSR_PMIP)],
>>> +                           riscv_iommu_hpm_irq_handler, IRQF_ONESHOT | IRQF_SHARED, NULL, iommu);
>>
>> Hmm, shared interrupts are tricky for PMUs, since perf requires any IRQ
>> handler touching a PMU is running on pmu->cpu, so you have to be very
>> careful about maintaining affinity and not letting anyone else change it
>> behind your back.
>>
>> The other thing is that if it really is shared, at this point you could
>> now be in riscv_iommu_pmu_handle_irq() dereferencing NULL.
> 
> Yes, the PMU IRQ line could be shared with the command queue, fault
> queue, and page-request queue.
> Could you please provide more tips on what is meant by "dereferencing
> NULL in riscv_iommu_pmu_handle_irq()"?
> I don't complete understand what needs to be done there.
> Thanks

In general it's not a great idea to register an IRQ handler before the 
data passed to that handler is initialised. What is pointed to by 
(&iommu->pmu)->reg + RISCV_IOMMU_REG_IOCOUNTOVF if the IRQ fires right 
now (and/or if CONFIG_DEBUG_SHIRQ ever gets fixed)? ;)

(OK, it's not *literally* NULL, but hey...)

Thanks,
Robin.

>>
>>> +     if (rc)
>>> +             return rc;
>>> +
>>> +     return riscv_iommu_pmu_init(&iommu->pmu, iommu->reg, dev_name(iommu->dev));
>>> +}
>>> +
>>> +/* HPM uninitialization */
>>> +static void riscv_iommu_hpm_disable(struct riscv_iommu_device *iommu)
>>> +{
>>> +     if (!(iommu->caps & RISCV_IOMMU_CAPABILITIES_HPM))
>>> +             return;
>>> +
>>> +     devm_free_irq(iommu->dev,
>>> +                   iommu->irqs[riscv_iommu_queue_vec(iommu, RISCV_IOMMU_IPSR_PMIP)],
>>> +                   iommu);
>>> +
>>> +     riscv_iommu_pmu_uninit(&iommu->pmu);
>>> +}
>>> +
>>>    /* Lookup and initialize device context info structure. */
>>>    static struct riscv_iommu_dc *riscv_iommu_get_dc(struct riscv_iommu_device *iommu,
>>>                                                 unsigned int devid)
>>> @@ -1596,6 +1652,9 @@ void riscv_iommu_remove(struct riscv_iommu_device *iommu)
>>>        riscv_iommu_iodir_set_mode(iommu, RISCV_IOMMU_DDTP_IOMMU_MODE_OFF);
>>>        riscv_iommu_queue_disable(&iommu->cmdq);
>>>        riscv_iommu_queue_disable(&iommu->fltq);
>>> +
>>> +     if (iommu->caps & RISCV_IOMMU_CAPABILITIES_HPM)
>>> +             riscv_iommu_pmu_uninit(&iommu->pmu);
>>>    }
>>>
>>>    int riscv_iommu_init(struct riscv_iommu_device *iommu)
>>> @@ -1635,6 +1694,10 @@ int riscv_iommu_init(struct riscv_iommu_device *iommu)
>>>        if (rc)
>>>                goto err_queue_disable;
>>>
>>> +     rc = riscv_iommu_hpm_enable(iommu);
>>> +     if (rc)
>>> +             goto err_hpm_disable;
>>> +
>>
>> I would leave this until after the whole IOMMU setup has succeeded. The
>> PMU is not critical to IOMMU operation, so at that point an error is not
>> fatal, it just means you don't get a PMU, thus there shouldn't need to
>> be any cleanup outside riscv_iommu_hpm_enable() itself.
> 
> Thanks for pointing this out. PMU failure shouldn't cause the entire
> IOMMU to fail, let me modify it in the next version.
> 
>>
>> Thanks,
>> Robin.
>>
>>>        rc = iommu_device_sysfs_add(&iommu->iommu, NULL, NULL, "riscv-iommu@%s",
>>>                                    dev_name(iommu->dev));
>>>        if (rc) {
>>> @@ -1653,6 +1716,8 @@ int riscv_iommu_init(struct riscv_iommu_device *iommu)
>>>    err_remove_sysfs:
>>>        iommu_device_sysfs_remove(&iommu->iommu);
>>>    err_iodir_off:
>>> +     riscv_iommu_hpm_disable(iommu);
>>> +err_hpm_disable:
>>>        riscv_iommu_iodir_set_mode(iommu, RISCV_IOMMU_DDTP_IOMMU_MODE_OFF);
>>>    err_queue_disable:
>>>        riscv_iommu_queue_disable(&iommu->fltq);
>>

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