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Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.
To:     Jason Gunthorpe <jgg@mellanox.com>
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From:   "Dey, Megha" <megha.dey@linux.intel.com>
Message-ID: <5bc05b74-536f-f72d-c406-18644436f11b@linux.intel.com>
Date:   Sun, 3 May 2020 15:31:39 -0700
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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
>