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    SubjectRe: [PATCH RFC 00/15] Add VFIO mediated device support and IMS support for the idxd driver.
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    On 5/8/2020 4:16 PM, Jason Gunthorpe wrote:
    > On Fri, May 08, 2020 at 01:47:10PM -0700, Raj, Ashok wrote:
    >
    >> Even when uaccel was under development, one of the options
    >> was to use VFIO as the transport, goal was the same i.e to keep
    >> the user space have one interface.
    >
    > I feel a bit out of the loop here, uaccel isn't in today's kernel is
    > it? I've heard about it for a while, it sounds very similar to RDMA,
    > so I hope they took some of my advice...

    It went into 5.7 kernel. drivers/misc/uacce. It looks char device exported with
    SVM support.

    >
    >> But the needs of generic user space application is significantly
    >> different from exporting a more functional device model to guest,
    >> which isn't full emulated device. which is why VFIO didn't make
    >> sense for native use.
    >
    > I'm not sure this is true. We've done these kinds of emulated SIOV
    > like things already and there is a huge overlap between what a generic
    > user application needs and what the VMM neds. Actually almost a
    > perfect subset except for interrupt remapping (which is quite
    > trivial).
    >
    > The things vfio focuses on, like groups and managing a real config
    > space just don't apply here.
    >
    >> And when we move things from VFIO which is already established
    >> as a general device model and accepted by multiple VMM's it gives
    >> instant footing without a whole redesign.
    >
    > Yes, I understand, but I think you need to get more people to support
    > this idea. From my standpoint this is taking secure lean VMMs and
    > putting emulation code back into them, except in a more dangerous
    > kernel location. This does not seem like a net win to me.
    >
    > You'd be much better to have some userspace library scheme instead of
    > being completely tied to a kernel interface for modularity.
    >
    >> When we move things from VFIO to uaccel to bolt on the functionality
    >> like VFIO, I suspect we would be moving code/functionality from VFIO
    >> to Uaccel. I don't know what the net gain would be.
    >
    > Most of VFIO functionality is already decomposed inside the kernel,
    > and you need most of it to do secure user access anyhow.
    >
    >> For mdev, would you agree we can keep the current architecture,
    >> and investigate moving some emulation code to user space (say even for
    >> standard vfio_pci) and then expand scope later.
    >
    > I won't hard NAK this, but I think you need more people to support
    > this general idea of more emulation code in the kernel to go ahead -
    > particularly since this is one of many future drivers along this
    > design.
    >
    > It would be good to hear from the VMM teams that this is what they
    > want (and why), for instance.
    >
    > Jason
    >

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