linux-kernel - Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver

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Date:   Fri, 7 Aug 2020 09:19:55 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
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        "Liu, Yi L" <yi.l.liu@...el.com>, "Lu, Baolu" <baolu.lu@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
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        "Williams, Dan J" <dan.j.williams@...el.com>,
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        "Hossain, Mona" <mona.hossain@...el.com>,
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        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and
 DEV-MSI support for the idxd driver

On Wed, Aug 05, 2020 at 07:22:58PM -0600, Alex Williamson wrote:

> If you see this as an abuse of the framework, then let's identify those
> specific issues and come up with a better approach.  As we've discussed
> before, things like basic PCI config space emulation are acceptable
> overhead and low risk (imo) and some degree of register emulation is
> well within the territory of an mdev driver.  

What troubles me is that idxd already has a direct userspace interface
to its HW, and does userspace DMA. The purpose of this mdev is to
provide a second direct userspace interface that is a little different
and trivially plugs into the virtualization stack.

I don't think VFIO should be the only entry point to
virtualization. If we say the universe of devices doing user space DMA
must also implement a VFIO mdev to plug into virtualization then it
will be alot of mdevs.

I would prefer to see that the existing userspace interface have the
extra needed bits for virtualization (eg by having appropriate
internal kernel APIs to make this easy) and all the emulation to build
the synthetic PCI device be done in userspace.

Not only is it better for security, it keeps things to one device
driver per device..

Jason