Re: [PATCH 3/7] vfio/pci: Add qcom-vfio-pci variant driver

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Oct 01, 2026 at 09:19:45AM +0200, Jose Ignacio Tornos Martinez wrote:

> Regarding platform_device_msi_init_and_alloc_irqs(), I agree that
> moving the driver to device MSI domains would be cleaner on the
> driver side. However, even with that change, I think the VM problem
> remains exactly the same: the device MSI domain in the VM would
> allocate virtual addr/data pairs, and the firmware's private interrupt
> controller still needs the physical host values.

Yes, I'm not saying it fixes the problem..

> Also, the VFIO variant driver scheme would not fully break if the
> driver moved to device MSI domains. The variant driver caches host
> MSI values from the MSI descriptor, regardless of the allocation
> method. The hook point may need adaptation, but the fundamental
> approach (host caches physical values, VM reads them) remains valid.

It breaks because VFIO will only cache *actual* MSI-X table entries,
it does not have visibility into anything that
platform_device_msi_init_and_alloc_irqs()

The only reason this works for you at all is because the driver
hackily copies the vectors from the real MSI-X table so it can look in
the "cache" to find their true physical versions.

Which is my general objection, I would like to see the driver to use
platform_device_msi_init_and_alloc_irqs() and don't want to get stuck
unable to do that because it would break this.

> Regarding the hypercall idea for device MSI domains, I think that
> would be an interesting direction worth exploring as a generic
> solution, but that's a multi-subsystem effort that will take time to
> design and land.

Yes, but it does actually solve the problem in all its forms..

> I completely agree that this could theoretically affect any device
> with private MSI registers. However, in practice, I'm only aware of
> this specific behavior in Qualcomm ath11k/ath12k firmware, where the
> device's interrupt controller requires the host physical MSI
> addresses to be programmed directly. 

Everyone else seems to know this stuff doesn't work for
virtualization and doesn't try to do something like that.

> It's also worth noting that the PCI reset support for these devices
> has already landed in linux-next (commit 290153d46d1a "PCI: Add
> device-specific reset for Qualcomm devices") [1], solving the
> device reset path for VFIO passthrough. This MSI series is the
> remaining piece, with both in place, these Qualcomm WiFi devices
> would be fully operational in VMs for the first time.

Why do people care so much about this? I always thought it was a bit
odd?

Jason