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

From: "Dey, Megha" <megha.dey@linux.intel.com>
To: Jason Gunthorpe <jgg@mellanox.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	Dave Jiang <dave.jiang@intel.com>, Vinod Koul <vkoul@kernel.org>,
	maz@kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>, Yi L Liu <yi.l.liu@intel.com>,
	baolu.lu@intel.com, "Tian, Kevin" <kevin.tian@intel.com>,
	Sanjay K Kumar <sanjay.k.kumar@intel.com>,
	"Luck, Tony" <tony.luck@intel.com>, Jing Lin <jing.lin@intel.com>,
	kwankhede@nvidia.com, eric.auger@redhat.com, parav@mellanox.com,
	dmaengine@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>,
	linux-pci@vger.kernel.org, KVM list <kvm@vger.kernel.org>
Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS support for the idxd driver.
Date: Sun, 3 May 2020 15:31:39 -0700	[thread overview]
Message-ID: <5bc05b74-536f-f72d-c406-18644436f11b@linux.intel.com> (raw)
In-Reply-To: <20200503222229.GE19158@mellanox.com>

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
> 