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

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Date:   Sat, 9 May 2020 09:21:13 -0300
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     "Raj, Ashok" <ashok.raj@...el.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Jiang, Dave" <dave.jiang@...el.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>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.

On Fri, May 08, 2020 at 05:09:09PM -0700, Raj, Ashok wrote:
> Hi Jason
> 
> On Fri, May 08, 2020 at 08:16:10PM -0300, 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...
> 
> I think since 5.7 maybe? drivers/misc/uacce. I don't think this is like
> RDMA, its just a plain accelerator. There is no connection management,
> memory registration or other things.. IB was my first job at Intel,
> but saying that i would be giving my age away :)

rdma was the first thing to do kernel bypass, all this stuff is like
rdma at some level.. I see this looks like the 'warp driver' stuff
redone

Wow, lots wrong here. Oh well.

> > putting emulation code back into them, except in a more dangerous
> > kernel location. This does not seem like a net win to me.
> 
> Its not a whole lot of emulation right? mdev are soft partitioned. There is
> just a single PF, but we can create a separate partition for the guest using
> PASID along with the normal BDF (RID). And exposing a consistent PCI like
> interface to user space you get everything else for free.
> 
> Yes, its not SRIOV, but giving that interface to user space via VFIO, we get 
> all of that functionality without having to reinvent a different way to do it.
> 
> vDPA went the other way, IRC, they went and put a HW implementation of what
> virtio is in hardware. So they sort of fit the model. Here the instance
> looks and feels like real hardware for the setup and control aspect.

VDPA and this are very similar, of course it depends on the exact HW
implementation.

Jason