Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS support for the idxd driver.

[Dey, Megha]

Hi Jason,

On 5/3/2020 3:22 PM, Jason Gunthorpe wrote:
> On Fri, May 01, 2020 at 03:31:51PM -0700, Dey, Megha wrote:
> > > > This has been my concern reviewing the implementation. IMS needs more
> > > > than one in-tree user to validate degrees of freedom in the api. I had
> > > > been missing a second "in-tree user" to validate the scope of the
> > > > flexibility that was needed.
> > >
> > > IMS is too narrowly specified.
> > >
> > > All platforms that support MSI today can support IMS. It is simply a
> > > way for the platform to give the driver an addr/data pair that triggers
> > > an interrupt when a posted write is performed to that pair.
> >
> > Well, yes and no. IMS requires interrupt remapping in addition to the
> > dynamic nature of IRQ allocation.
>
> You've mentioned remapping a few times, but I really can't understand
> why it has anything to do with platform_msi or IMS..

So after some internal discussions, we have concluded that IMS has no 
linkage with Interrupt remapping, IR is just a platform concept. IMS is 
just a name Intel came up with, all it really means is device managed 
addr/data writes to generate interrupts. Technically we can call 
something IMS even if device has its own location to store interrupts in 
non-pci standard mechanism, much like platform-msi indeed. We simply 
need to extend platform-msi to its address some of its shortcomings: 
increase number of interrupts to > 2048, enable dynamic allocation of 
interrupts, add mask/unmask callbacks in addition to write_msg etc.
FWIW, even MSI can be IMS with rules on how to manage the addr/data 
writes following pci sig .. its just that.

I will be sending out an email shortly outlining the new design for IMS 
(A.K.A platform-msi part 2) and what are the improvements we want to add 
to the already existing platform-msi infrastructure.

Thank you so much for your comments, it helped us iron out some of these 
details :)

> Jason