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Date:   Fri, 1 May 2020 15:31:51 -0700
From:   "Dey, Megha" <>
To:     Jason Gunthorpe <>,
        Dan Williams <>
Cc:     Dave Jiang <>, Vinod Koul <>,, Bjorn Helgaas <>,
        "Rafael J. Wysocki" <>,
        Greg KH <>,
        Thomas Gleixner <>,
        "H. Peter Anvin" <>,
        Alex Williamson <>,
        Jacob jun Pan <>,
        "Raj, Ashok" <>, Yi L Liu <>,, "Tian, Kevin" <>,
        Sanjay K Kumar <>,
        "Luck, Tony" <>, Jing Lin <>,,,,,
        Linux Kernel Mailing List <>,
        X86 ML <>,,
        KVM list <>
Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.

Hi Jason,

On 4/23/2020 12:18 PM, Jason Gunthorpe wrote:
> On Wed, Apr 22, 2020 at 02:24:11PM -0700, Dan Williams wrote:
>> On Tue, Apr 21, 2020 at 4:55 PM Jason Gunthorpe <> wrote:
>>> On Tue, Apr 21, 2020 at 04:33:46PM -0700, Dave Jiang wrote:
>>>> The actual code is independent of the stage 2 driver code submission that adds
>>>> support for SVM, ENQCMD(S), PASID, and shared workqueues. This code series will
>>>> support dedicated workqueue on a guest with no vIOMMU.
>>>> A new device type "mdev" is introduced for the idxd driver. This allows the wq
>>>> to be dedicated to the usage of a VFIO mediated device (mdev). Once the work
>>>> queue (wq) is enabled, an uuid generated by the user can be added to the wq
>>>> through the uuid sysfs attribute for the wq.  After the association, a mdev can
>>>> be created using this UUID. The mdev driver code will associate the uuid and
>>>> setup the mdev on the driver side. When the create operation is successful, the
>>>> uuid can be passed to qemu. When the guest boots up, it should discover a DSA
>>>> device when doing PCI discovery.
>>> I'm feeling really skeptical that adding all this PCI config space and
>>> MMIO BAR emulation to the kernel just to cram this into a VFIO
>>> interface is a good idea, that kind of stuff is much safer in
>>> userspace.
>>> Particularly since vfio is not really needed once a driver is using
>>> the PASID stuff. We already have general code for drivers to use to
>>> attach a PASID to a mm_struct - and using vfio while disabling all the
>>> DMA/iommu config really seems like an abuse.
>>> A /dev/idxd char dev that mmaps a bar page and links it to a PASID
>>> seems a lot simpler and saner kernel wise.
>>>> The mdev utilizes Interrupt Message Store or IMS[3] instead of MSIX for
>>>> interrupts for the guest. This preserves MSIX for host usages and also allows a
>>>> significantly larger number of interrupt vectors for guest usage.
>>> I never did get a reply to my earlier remarks on the IMS patches.
>>> The concept of a device specific addr/data table format for MSI is not
>>> Intel specific. This should be general code. We have a device that can
>>> use this kind of kernel capability today.
>> 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.

> This is different from the other interrupt setup flows which are
> tightly tied to the PCI layer. Here the driver should simply ask for
> interrupts.
> Ie the entire IMS API to the driver should be something very simple
> like:
>   struct message_irq
>   {
>     uint64_t addr;
>     uint32_t data;
>   };
>   struct message_irq *request_message_irq(
>      struct device *, irq_handler_t handler, unsigned long flags,
>      const char *name, void *dev);
> And the plumbing underneath should setup the irq chips and so forth as
> required.

yes, this seems correct.
> Jason

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