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

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Date:   Tue, 21 Apr 2020 20:54:42 -0300
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     vkoul@...nel.org, megha.dey@...ux.intel.com, maz@...nel.org,
        bhelgaas@...gle.com, rafael@...nel.org, gregkh@...uxfoundation.org,
        tglx@...utronix.de, hpa@...or.com, alex.williamson@...hat.com,
        jacob.jun.pan@...el.com, ashok.raj@...el.com, yi.l.liu@...el.com,
        baolu.lu@...el.com, kevin.tian@...el.com, sanjay.k.kumar@...el.com,
        tony.luck@...el.com, jing.lin@...el.com, dan.j.williams@...el.com,
        kwankhede@...dia.com, eric.auger@...hat.com, parav@...lanox.com,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-pci@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.

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.

Jason