Re: [PATCH RFC/RFT] vfio/pci: Create feature to disable MSI virtualization

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Aug 13 2024 at 13:30, Jason Gunthorpe wrote:
> On Mon, Aug 12, 2024 at 10:59:12AM -0600, Alex Williamson wrote:
>> vfio-pci has always virtualized the MSI address and data registers as
>> MSI programming is performed through the SET_IRQS ioctl.  Often this
>> virtualization is not used, and in specific cases can be unhelpful.
>> 
>> One such case where the virtualization is a hinderance is when the
>> device contains an onboard interrupt controller programmed by the guest
>> driver.  Userspace VMMs have a chance to quirk this programming,
>> injecting the host physical MSI information, but only if the userspace
>> driver can get access to the host physical address and data registers.
>> 
>> This introduces a device feature which allows the userspace driver to
>> disable virtualization of the MSI capability address and data registers
>> in order to provide read-only access the the physical values.
>
> Personally, I very much dislike this. Encouraging such hacky driver
> use of the interrupt subsystem is not a good direction. Enabling this
> in VMs will further complicate fixing the IRQ usages in these drivers
> over the long run.
>
> If the device has it's own interrupt sources then the device needs to
> create an irq_chip and related and hook them up properly. Not hackily
> read the MSI-X registers and write them someplace else.
>
> Thomas Gleixner has done alot of great work recently to clean this up.
>
> So if you imagine the driver is fixed, then this is not necessary.

Yes. I looked at the at11k driver when I was reworking the PCI/MSI
subsystem and that's a perfect candidate for a proper device specific
interrupt domain to replace the horrible MSI hackery it has.

> Howver, it will still not work in a VM. Making IMS and non-MSI
> interrupt controlers work within VMs is still something that needs to
> be done.

Sure, but we really want to do that in a generic way and not based on ad
hoc workarounds.

Did the debate around this go anywhere?

Thanks,

        tglx