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

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Date:   Thu, 3 Jun 2021 15:48:23 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, Jason Wang <jasowang@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Thu, Jun 03, 2021 at 02:49:56AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Thursday, June 3, 2021 12:59 AM
> > 
> > On Wed, Jun 02, 2021 at 04:48:35PM +1000, David Gibson wrote:
> > > > > 	/* Bind guest I/O page table  */
> > > > > 	bind_data = {
> > > > > 		.ioasid	= gva_ioasid;
> > > > > 		.addr	= gva_pgtable1;
> > > > > 		// and format information
> > > > > 	};
> > > > > 	ioctl(ioasid_fd, IOASID_BIND_PGTABLE, &bind_data);
> > > >
> > > > Again I do wonder if this should just be part of alloc_ioasid. Is
> > > > there any reason to split these things? The only advantage to the
> > > > split is the device is known, but the device shouldn't impact
> > > > anything..
> > >
> > > I'm pretty sure the device(s) could matter, although they probably
> > > won't usually.
> > 
> > It is a bit subtle, but the /dev/iommu fd itself is connected to the
> > devices first. This prevents wildly incompatible devices from being
> > joined together, and allows some "get info" to report the capability
> > union of all devices if we want to do that.
> 
> I would expect the capability reported per-device via /dev/iommu. 
> Incompatible devices can bind to the same fd but cannot attach to
> the same IOASID. This allows incompatible devices to share locked
> page accounting.

Yeah... I'm not convinced that everything relevant here can be
reported per-device.  I think we may have edge cases where
combinations of devices have restrictions that individual devices in
the set do not.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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