Re: pcie-iproc-msi.c: Bug in Multi-MSI support?

[Ray Jui <ray.jui@broadcom.com>]

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On 5/20/2021 7:22 AM, Sandor Bodo-Merle wrote:
> On Thu, May 20, 2021 at 4:05 PM Pali Rohár <pali@kernel.org> wrote:
>>
>> Hello!
>>
>> On Thursday 20 May 2021 15:47:46 Sandor Bodo-Merle wrote:
>>> Hi Pali,
>>>
>>> thanks for catching this - i dig up the followup fixup commit we have
>>> for the iproc multi MSI (it was sent to Broadcom - but unfortunately
>>> we missed upstreaming it).
>>>
>>> Commit fc54bae28818 ("PCI: iproc: Allow allocation of multiple MSIs")
>>> failed to reserve the proper number of bits from the inner domain.
>>> We need to allocate the proper amount of bits otherwise the domains for
>>> multiple PCIe endpoints may overlap and freeing one of them will result
>>> in freeing unrelated MSI vectors.
>>>
>>> Fixes: fc54bae28818 ("PCI: iproc: Allow allocation of multiple MSIs")
>>> ---
>>>  drivers/pci/host/pcie-iproc-msi.c | 8 ++++----
>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git drivers/pci/host/pcie-iproc-msi.c drivers/pci/host/pcie-iproc-msi.c
>>> index 708fdb1065f8..a00492dccb74 100644
>>> --- drivers/pci/host/pcie-iproc-msi.c
>>> +++ drivers/pci/host/pcie-iproc-msi.c
>>> @@ -260,11 +260,11 @@ static int iproc_msi_irq_domain_alloc(struct
>>> irq_domain *domain,
>>>
>>>         mutex_lock(&msi->bitmap_lock);
>>>
>>> -       /* Allocate 'nr_cpus' number of MSI vectors each time */
>>> +       /* Allocate 'nr_irqs' multiplied by 'nr_cpus' number of MSI
>>> vectors each time */
>>>         hwirq = bitmap_find_next_zero_area(msi->bitmap, msi->nr_msi_vecs, 0,
>>> -                                          msi->nr_cpus, 0);
>>> +                                          msi->nr_cpus * nr_irqs, 0);
>>
>> I'm not sure if this construction is correct. Multi-MSI interrupts needs
>> to be aligned to number of requested interrupts. So if wifi driver asks
>> for 32 Multi-MSI interrupts then first allocated interrupt number must
>> be dividable by 32.
>>
> 
> Ahh - i guess you are right. In our internal engineering we always
> request 32 vectors.
> IIRC the multiply by "nr_irqs" was added for iqr affinity to work correctly.
> 

May I ask which platforms are you guys running this driver on? Cygnus or
Northstar? Not that it matters, but just out of curiosity.

Let me start by explaining how MSI support works in this driver, or more
precisely, for all platforms that support this iProc based event queue
MSI scheme:

In iProc PCIe core, each MSI group is serviced by a GIC interrupt
(hwirq) and a dedicated event queue (event queue is paired up with
hwirq).  Each MSI group can support up to 64 MSI vectors. Note 64 is the
depth of the event queue.

The number of MSI groups varies between different iProc SoCs. The total
number of CPU cores also varies. To support MSI IRQ affinity, we
distribute GIC interrupts across all available CPUs.  MSI vector is
moved from one GIC interrupt to another to steer to the target CPU.

Assuming:
The number of MSI groups (the number of total hwirq for this PCIe
controller) is M
The number of CPU cores is N
M is always a multiple of N (we ensured that in the setup function)

Therefore:
Total number of raw MSI vectors = M * 64
Total number of supported MSI vectors = (M * 64) / N

I guess I'm not too clear on what you mean by "multi-MSI interrupts
needs to be aligned to number of requested interrupts.". Would you be
able to plug this into the above explanation so we can have a more clear
understanding of what you mean here?

In general, I don't see much issue of allocating 'msi->nr_cpus *
nr_irqs' here as long as we can still meet the affinity distribution
requirement as mentioned above.

Example in the dw PCIe driver does the following for reserving in the
bitmap:

bitmap_find_free_region(pp->msi_irq_in_use, pp->num_vectors,
order_base_2(nr_irqs));

>>>         if (hwirq < msi->nr_msi_vecs) {
>>> -               bitmap_set(msi->bitmap, hwirq, msi->nr_cpus);
>>> +               bitmap_set(msi->bitmap, hwirq, msi->nr_cpus * nr_irqs);
>>
>> And another issue is that only power of 2 interrupts for Multi-MSI can
>> be allocated. Otherwise one number may be allocated to more devices.
>>
>> But I'm not sure how number of CPUs affects it as other PCIe controller
>> drivers do not use number of CPUs.

As I explained above, we definitely need to take nr_cpus into account,
since we cannot move around the physical interrupt between CPUs.
Instead, we dedicate each physical interrupt to the CPU and service the
MSI across different event queues accordingly when user change irq affinity.

>>
>> Other drivers are using bitmap_find_free_region() function with
>> order_base_2(nr_irqs) as argument.
>>

Yes we should do that too.

>> I hope that somebody else more skilled with MSI interrupts look at these
>> constructions if are correct or needs more rework.
>>
> 
> I see the point - i'll ask also in our engineering department ...
> 
>>>         } else {
>>>                 mutex_unlock(&msi->bitmap_lock);
>>>                 return -ENOSPC;
>>> @@ -292,7 +292,7 @@ static void iproc_msi_irq_domain_free(struct
>>> irq_domain *domain,
>>>         mutex_lock(&msi->bitmap_lock);
>>>
>>>         hwirq = hwirq_to_canonical_hwirq(msi, data->hwirq);
>>> -       bitmap_clear(msi->bitmap, hwirq, msi->nr_cpus);
>>> +       bitmap_clear(msi->bitmap, hwirq, msi->nr_cpus * nr_irqs);
>>>
>>>         mutex_unlock(&msi->bitmap_lock);
>>>
>>>
>>> On Thu, May 20, 2021 at 2:04 PM Pali Rohár <pali@kernel.org> wrote:
>>>>
>>>> Hello!
>>>>
>>>> I think there is a bug in pcie-iproc-msi.c driver. It declares
>>>> Multi MSI support via MSI_FLAG_MULTI_PCI_MSI flag, see:
>>>>
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/pcie-iproc-msi.c?h=v5.12#n174
>>>>
>>>> but its iproc_msi_irq_domain_alloc() function completely ignores nr_irqs
>>>> argument when allocating interrupt numbers from bitmap, see:
>>>>
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/pcie-iproc-msi.c?h=v5.12#n246
>>>>
>>>> I think this this is incorrect as alloc callback should allocate nr_irqs
>>>> multi interrupts as caller requested. All other drivers with Multi MSI
>>>> support are doing it.
>>>>
>>>> Could you look at it?

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