Re: [PATCH v3 5/9] perf/cxl: Accept an overflow interrupt on MSI message number 0

On 8/2/26 10:57 PM, Richard Cheng wrote:
> On Fri, Jul 31, 2026 at 04:28:23PM +0800, Dave Jiang wrote:
>> cxl_pmu_probe() rejects the PMU when info->msi_vec <= 0, but that field
>> holds the MSI/MSI-X message number the device signals overflow on. The
>> field is 0-based, and -1 means no interrupt support. Message number 0 is
>> valid and pci_irq_vector() takes a 0-based index, so a compliant device
>> signalling on the first vector fails to probe.
>>
>> Reject only the no-interrupt case, matching how the CXL mailbox and event
>> interrupts handle it.
>>
>> Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
>> Reported-by: [email protected]
>> Closes: https://sashiko.dev/#/patchset/[email protected]?part=1
>> Assisted-by: Claude:claude-opus-4-8
>> Reviewed-by: Jonathan Cameron <[email protected]>
>> Reviewed-by: Davidlohr Bueso <[email protected]>
>> Signed-off-by: Dave Jiang <[email protected]>
>> ---
>> v3:
>> - Rebase on the preceding patch, so this tests info->msi_vec rather than
>>   info->irq. The fix is unchanged and the hunk is still one line.
>> - Not guarding against pci_irq_vector()'s INTx fallback for vector 0, which
>>   it returns when neither MSI nor MSI-X is enabled. A v3 draft added a
>>   pci_dev_msi_enabled() check for that, but a modern CXL device wiring up
>>   INTx is not a realistic prospect, and the consequence otherwise is a PMU
>>   that counts nothing rather than anything unsafe. Left alone to keep this
>>   a one-liner.
>> ---
>>  drivers/perf/cxl_pmu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
>> index 56e6ae7bebf7..c9e30cb149df 100644
>> --- a/drivers/perf/cxl_pmu.c
>> +++ b/drivers/perf/cxl_pmu.c
>> @@ -875,7 +875,7 @@ static int cxl_pmu_probe(struct device *dev)
>>  		.capabilities = PERF_PMU_CAP_NO_EXCLUDE,
>>  	};
>>  
>> -	if (info->msi_vec <= 0)
>> +	if (info->msi_vec < 0)
>>  		return -EINVAL;
>>
> 
> Hi Dave,
> 
> I agree that 0 is a legal message number, but do we need to check whether
> MSI is actually enabled ?

I actually dropped the check and added notes so sashiko doesn't freak out. I don't think it's necessary given there's not likely to have devices with INTX support.


> 
> something like
> """
> if (info->msi_vec < 0 || (!pdev->msi_enabled && !pdev->msix_enabled))
>     return -EINVAL;
> """

I can add that if you still think it's needed. The one thing I can think of is someone pass in no_msi at the kernel commandline.  

> 
> Best regards,
> Richard Cheng
> 
> 
> 
> 
>>  	rc = pci_irq_vector(pdev, info->msi_vec);
>> -- 
>> 2.55.0
>>
>>