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

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Date:   Tue, 15 Jun 2021 20:59:28 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.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

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.

Jason