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

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Date:   Sun, 3 May 2020 15:31:39 -0700
From:   "Dey, Megha" <megha.dey@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>, maz@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>, Yi L Liu <yi.l.liu@...el.com>,
        baolu.lu@...el.com, "Tian, Kevin" <kevin.tian@...el.com>,
        Sanjay K Kumar <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>, Jing Lin <jing.lin@...el.com>,
        kwankhede@...dia.com, eric.auger@...hat.com, parav@...lanox.com,
        dmaengine@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, linux-pci@...r.kernel.org,
        KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.

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
>