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

[Jason Gunthorpe]

On Sun, May 03, 2020 at 03:31:39PM -0700, Dey, Megha wrote:
> 
> 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.

Sounds right to me

Persumably you still need a way for the driver, eg vfio, to ensure a
MSI is remappable, but shouldn't that be exactly the same way as done
in normal PCI MSI today?

> FWIW, even MSI can be IMS with rules on how to manage the addr/data writes
> following pci sig .. its just that.

Yep, IMHO, our whole handling of MSI is very un-general sometimes..

I thought the msi_domain stuff that some platforms are using is a way
to improve on that? You might find that updating x86 to use msi_domain
might be helpful in this project???

Jason