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

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