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:   Mon, 17 Aug 2020 02:12:44 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "Dey, Megha" <megha.dey@...el.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>,
        "Raj, Ashok" <ashok.raj@...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>,
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        "parav@...lanox.com" <parav@...lanox.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
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        "Ortiz, Samuel" <samuel.ortiz@...el.com>,
        "Hossain, Mona" <mona.hossain@...el.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 v2 00/18] Add VFIO mediated device support and DEV-MSI
 support for the idxd driver

> From: Jason Gunthorpe
> Sent: Friday, August 14, 2020 9:35 PM
> 
> On Mon, Aug 10, 2020 at 07:32:24AM +0000, Tian, Kevin wrote:
> 
> > > 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.
> >
> > In the end what decides the direction is the amount of changes that
> > we have to put in kernel, not whether we call it 'emulation'.
> 
> No, this is not right. The decision should be based on what will end
> up more maintable in the long run.
> 
> Yes it would be more code to dis-aggregate some of the things
> currently only bundled as uAPI inside VFIO (eg your vSVA argument
> above) but once it is disaggregated the maintability of the whole
> solution will be better overall, and more drivers will be able to use
> this functionality.
> 

Disaggregation is an orthogonal topic to the main divergence in 
this thread, which is passthrough vs. userspace DMA. I gave detail
explanation about the difference between the two in last reply.
the possibility of dis-aggregating something between passthrough
frameworks (e.g. VFIO and vDPA) is not the reason for growing 
every userspace DMA framework to be a passthrough framework.
Doing that is instead hurting maintainability in general...

Thanks
Kevin