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Date:   Fri, 8 May 2020 16:52:57 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Jason Gunthorpe <jgg@...lanox.com>,
        "Raj, Ashok" <ashok.raj@...el.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "megha.dey@...ux.intel.com" <megha.dey@...ux.intel.com>,
        "maz@...nel.org" <maz@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "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>,
        "Lin, Jing" <jing.lin@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "kvm@...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 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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