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

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Date:   Tue, 3 Sep 2019 17:16:16 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     John Garry <john.garry@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Linux PCI <linux-pci@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>,
        "luojiaxing@...wei.com" <luojiaxing@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: PCI/kernel msi code vs GIC ITS driver conflict?

Hi John,

On 03/09/2019 15:09, John Garry wrote:
> Hi Marc, Bjorn, Thomas,
> 
> 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?

> 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...

> 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. 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? If
there's a Linux driver somewhere, I can always hack something in
emulation and find out...

Thanks,

	M.
-- 
Jazz is not dead, it just smells funny...