Re: [PATCH RFC/RFT] vfio/pci-quirks: Quirk for ath wireless

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Aug 12, 2024 at 11:00:40AM -0600, Alex Williamson wrote:
> These devices have an embedded interrupt controller which is programmed
> with guest physical MSI address/data, which doesn't work.  We need
> vfio-pci kernel support to provide a device feature which disables
> virtualization of the MSI capability registers.  Then we can do brute
> force testing for writes matching the MSI address, from which we can
> infer writes of the MSI data, replacing each with host physical values.
> 
> This has only been tested on ath11k (0x1103), ath12k support is
> speculative and requires testing.  Note that Windows guest drivers make
> use of multi-vector MSI which requires interrupt remapping support in
> the host.

The way it is really supposed to work, is that the guest itself
controls/knows the MSI addr/data pairs and the interrupt remapping HW
makes that delegation safe since all the interrupt processing will be
qualified by the RID.

Then the guest can make up the unique interrupts for MSI and any
internal "IMS" sources and we just let the guest directly write the
MSI/MSI-X and any IMS values however it wants.

This hackery to capture and substitute the IMS programming is neat and
will solve this one device, but there are more IMS style devices in
the pipeline than will really need a full solution.

> + * The Windows driver makes use of multi-vector MSI, where our sanity test
> + * of the MSI data value must then mask off the vector offset for comparison
> + * and add it back to the host base data value on write.

But is that really enough? If the vector offset is newly created then
that means the VM built a new interrupt that needs setup to be routed
into the VM?? Is that why you say it "requires interrupt remapping
support" because that setup is happening implicitly on x86?

It looks like Windows is acting as I said Linux should, with a
"irq_chip" and so on to get the unique interrupt source a proper
unique addr/data pair...

Jason