linux-kernel - RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Date:   Fri, 7 May 2021 08:09:00 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     David Gibson <david@...son.dropbear.id.au>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        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>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Alexey Kardashevskiy" <aik@...abs.ru>
Subject: RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, May 5, 2021 1:13 AM
> 
> On Wed, Apr 28, 2021 at 06:58:19AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, April 28, 2021 1:12 AM
> > >
> > [...]
> > > > As Alex says, if this line fails because of the group restrictions,
> > > > that's not great because it's not very obvious what's gone wrong.
> > >
> > > Okay, that is fair, but let's solve that problem directly. For
> > > instance netlink has been going in the direction of adding a "extack"
> > > from the kernel which is a descriptive error string. If the failing
> > > ioctl returned the string:
> > >
> > >   "cannot join this device to the IOASID because device XXX in the
> > >    same group #10 is in use"
> > >
> > > Would you agree it is now obvious what has gone wrong? In fact would
> > > you agree this is a lot better user experience than what applications
> > > do today even though they have the group FD?
> > >
> >
> > Currently all the discussions are around implicit vs. explicit uAPI semantics
> > on the group restriction. However if we look beyond group the implicit
> > semantics might be inevitable when dealing with incompatible iommu
> > domains. An existing example of iommu incompatibility is IOMMU_
> > CACHE.
> 
> I still think we need to get rid of these incompatibilities
> somehow. Having multiple HW incompatible IOASID in the same platform
> is just bad all around.
> 
> When modeling in userspace IOMMU_CACHE sounds like it is a property of
> each individual IOASID, not an attribute that requires a new domain.

sure. the iommu domain is an kernel-internal concept. The userspace 
should focus everything on IOASID.

> 
> People that want to create cache bypass IOASID's should just ask for
> that that directly.
> 

Yes, in earlier discussion we agreed on a scheme that ioasid module
will return an error to userspace indicating incompatibility detected
when binding a device to ioasid then the userspace should create 
a new IOASID for this device. This has to be done 'explicitly'. 

When I used it as the example for 'implicit semantics" is that the kernel 
won't create another group-like object to contain devices with compatible 
attributes and 'explicitly' manage it in uAPI like group_fd. If we anyway
rely on the userspace to have more intelligence on those hardware
restrictions, it's little sense to only explicitly handle group_fd in uAPI.

Thanks
Kevin