linux-kernel - RE: [RFC] /dev/ioasid uAPI proposal

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Message-ID: <MWHPR11MB1886298A9D202A966F1D4DA68C0F9@MWHPR11MB1886.namprd11.prod.outlook.com>
Date:   Wed, 16 Jun 2021 00:02:19 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Jonathan Corbet" <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "David Gibson" <david@...son.dropbear.id.au>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: RE: [RFC] /dev/ioasid uAPI proposal

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, June 16, 2021 7:59 AM
> 
> On Tue, Jun 15, 2021 at 11:56:28PM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, June 16, 2021 7:41 AM
> > >
> > > On Tue, Jun 15, 2021 at 11:09:37PM +0000, Tian, Kevin wrote:
> > >
> > > > which information can you elaborate? This is the area which I'm not
> > > > familiar with thus would appreciate if you can help explain how this
> > > > bus specific information is utilized within the attach function or
> > > > sometime later.
> > >
> > > This is the idea that the device driver needs to specify which bus
> > > specific protocol it uses to issue DMA's when it attaches itself to an
> > > IOASID. For PCI:
> >
> > What about defining some general attributes instead of asking iommu
> > fd to understand those bus specific detail?
> 
> I prefer the API be very clear and intent driven, otherwise things
> just get confused.
> 
> The whole WBINVD/no-snoop discussion I think is proof of that :\
> 
> > from iommu p.o.v there is no difference from last one. In v2 the device
> > driver just needs to communicate the PASID virtualization policy at
> > device binding time,
> 
> I want it documented in the kernel source WTF is happening, because
> otherwise we are going to be completely lost in a few years. And your
> RFC did have device driver specific differences here
> 
> > > The device knows what it is going to do, we need to convey that to the
> > > IOMMU layer so it is prepared properly.
> >
> > Yes, but it's not necessarily to have iommu fd understand bus specific
> > attributes. In the end when /dev/iommu uAPI calls iommu layer interface,
> > it's all bus agnostic.
> 
> Why not? Just put some inline wrappers to translate the bus specific
> language to your generic language if that is what makes the most
> sense.
> 

I can do this. Thanks