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

[Jason Gunthorpe <jgg@ziepe.ca>]

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.

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.

Jason