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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Message-ID: <MWHPR11MB16456D49F2F2E9646F0841488C5F0@MWHPR11MB1645.namprd11.prod.outlook.com>
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>,
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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