Re: PCI/kernel msi code vs GIC ITS driver conflict?

[Andrew Murray]

On Wed, Sep 04, 2019 at 09:56:51AM +0100, John Garry wrote:
> On 03/09/2019 17:16, Marc Zyngier wrote:
> > Hi John,
> > 
> > On 03/09/2019 15:09, John Garry wrote:
> > > Hi Marc, Bjorn, Thomas,
> 
> Hi Marc,
> 
> > > 
> > > We've come across a conflict with the kernel/pci msi code and GIC ITS
> > > driver on our arm64 system, whereby we can't unbind and re-bind a PCI
> > > device driver under special conditions. I'll explain...
> > > 
> > > Our PCI device support 32 MSIs. The driver attempts to allocate msi
> > > vectors with min msi=17, max msi = 32, and affd.pre vectors = 16. For
> > > our test we make nr_cpus = 1 (just anything less than 16).
> > 
> > Just to confirm: this PCI device is requiring Multi-MSI, right? As
> > opposed to MSI-X?
> 
> Right, Multi-MSI.
> 
> > 
> > > We find that the pci/kernel msi code gives us 17 vectors, but the GIC
> > > ITS code reserves 32 lpi maps in its_irq_domain_alloc(). The problem
> > > then occurs when unbinding the driver in its_irq_domain_free() call,
> > > where we only clear bits for 17 vectors. So if we unbind the driver and
> > > then attempt to bind again, it fails.
> > 
> > Is this device, by any chance, sharing its requested-id with another
> > device? By being behind a bridge of some sort?There is some code to
> > deal with it, but I'm not sure it has ever been verified in anger...
> 
> It's a RC iEP and there should be no requested-id sharing:
> 
> root@ubuntu:/home/john#  lspci -s 74:02.0 -v
> 74:02.0 Serial Attached SCSI controller: Huawei Technologies Co., Ltd.
> HiSilicon SAS 3.0 HBA (rev 20)
> Flags: bus master, fast devsel, latency 0, IRQ 23, NUMA node 0
> Memory at a2000000 (32-bit, non-prefetchable) [size=32K]
> Capabilities: [40] Express Root Complex Integrated Endpoint, MSI 00
> Capabilities: [80] MSI: Enable+ Count=32/32 Maskable+ 64bit+
> Capabilities: [b0] Power Management version 3
> Kernel driver in use: hisi_sas_v3_hw
> 
> > 
> > > Where the fault lies, I can't say. Maybe the kernel msi code should
> > > always give power of 2 vectors - as I understand, the PCI spec mandates
> > > this. Or maybe the GIC ITS driver has a problem in the free path, as
> > > above. Or maybe the PCI driver should not be allowed to request !power
> > > of 2 min/max vectors.
> > > 
> > > Opinion?
> > 
> > My hunch is that it is an ITS driver bug: the PCI layer is allowed to
> > give any number of MSIs to an endpoint driver, as long as they match the
> > requirements of the allocation for Multi-MSI.
> 
> I would tend to say that, but isn't the requirement to allocate power of 2
> msi vectors, which doesn't seem to be enforced in the kernel msi layer?

For a PCI device that supports MSI but not MSI-X - my understanding is that
pci_alloc_irq_vectors_affinity and pci_alloc_irq_vectors will request *from
the device* a power of 2 msi vectors between the min and max given by the
driver - msi_setup_entry rounds up to nearest power of 2.

However this doesn't guarantee that pci_alloc_irq_vectors will return a
power of 2. For example if you set maxvec to 17, then it will request
32 from the device and pci_alloc_irq_vectors will return 17 (i.e. it satisfies
your request by over allocating, but still gives you what you asked for).

I'm not yet familiar with ITS, however if it is reserving 32 yet you only
clear 17, then there is mismatch between the number actually reserved from
the hardware, and the value returned from pci_alloc_irq_vectors.

(It looks like its_alloc_device_irq rounds up to the nearest power of 2).

Thanks,

Andrew Murray

> 
>  That's the responsibility
> > of the ITS driver. If unbind/bind fails, it means that somehow we've
> > missed the freeing of the LPIs, which isn't good.
> > 
> > Is the device common enough that I can try and reproduce the issue?
> 
> No, it's integrated into the hi1620 SoC found in the D06 dev board only, but
> I don't think that there is anything special about this HW.
> 
> If
> > there's a Linux driver somewhere, I can always hack something in
> > emulation and find out...
> 
> Ok, the interrupt allocation for this particular driver in this test is in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/hisi_sas/hisi_sas_v3_hw.c#n2393
> 
> Cheers,
> John
> 
> > 
> > Thanks,
> > 
> > 	M.
> > 
> 
>