[PATCH] vfio/pci: Fix INTx handling on legacy DisINTx- PCI devices

[Timothy Pearson]

----- Original Message -----
> From: "Alex Williamson" <alex.williamson at redhat.com>
> To: "Timothy Pearson" <tpearson at raptorengineering.com>
> Cc: "kvm" <kvm at vger.kernel.org>, "linuxppc-dev" <linuxppc-dev at lists.ozlabs.org>
> Sent: Monday, September 22, 2025 11:01:43 AM
> Subject: Re: [PATCH] vfio/pci: Fix INTx handling on legacy DisINTx- PCI devices

> On Sat, 20 Sep 2025 14:25:03 -0500 (CDT)
> Timothy Pearson <tpearson at raptorengineering.com> wrote:
>> Personally, I'd argue that such old devices were intended to work
>> with much slower host systems, therefore the slowdown probably
>> doesn't matter vs. being more correct in terms of interrupt handling.
>>  In terms of general kernel design, my understanding has always been
>> is that best practice is to always mask, disable, or clear a level
>> interrupt before exiting the associated IRQ handler, and the current
>> design seems to violate that rule.  In that context, I'd personally
>> want to see an argument as to why echewing this traditional IRQ
>> handler design is beneficial enough to justify making the VFIO driver
>> dependent on platform-specific behavior.
> 
> Yep, I kind of agree.  The unlazy flag seems to provide the more
> intended behavior.  It moves the irq chip masking into the fast path,
> whereas it would have been asynchronous on a subsequent interrupt
> previously, but the impact is only to ancient devices operating in INTx
> mode, so as long as we can verify those still work on both ppc and x86,
> I don't think it's worth complicating the code to make setting the
> unlazy flag conditional on anything other than the device support.
> 
> Care to send out a new version documenting the actual sequence fixed by
> this change and updating the code based on this thread?  Note that we
> can test non-pci2.3 mode for any device/driver that supports INTx using
> the nointxmask=1 option for vfio-pci and booting a linux guest with
> pci=nomsi.  Thanks,
> 
> Alex

Sure, I can update the commit message easily enough, but I must have missed something in regard to a needed code update.  The existing patch only sets unlazy for non-PCI 2.3 INTX devices, and as I understand it that's the behavior we have both agreed on at this point?

I've tested this on ppc64el and it works quite well, repairing the broken behavior where the guest would receive exactly one interrupt on the legacy PCI device per boot.  I don't have amd64 systems available to test on, however.

Thanks!