linux-kernel - Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach

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Date:   Tue, 15 Feb 2022 09:58:13 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Stuart Yoder <stuyoder@...il.com>, rafael@...nel.org,
        David Airlie <airlied@...ux.ie>, linux-pci@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Will Deacon <will@...nel.org>, Ashok Raj <ashok.raj@...el.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v1 3/8] iommu: Extend iommu_at[de]tach_device() for
 multi-device groups

On Mon, Feb 14, 2022 at 11:46:26AM -0400, Jason Gunthorpe wrote:
> On Mon, Feb 14, 2022 at 03:18:31PM +0000, Robin Murphy wrote:
> 
> > Arguably, iommu_attach_device() could be renamed something like
> > iommu_attach_group_for_dev(), since that's effectively the semantic that all
> > the existing API users want anyway (even VFIO at the high level - the group
> > is the means for the user to assign their GPU/NIC/whatever device to their
> > process, not the end in itself). That's just a lot more churn.
> 
> Right

Okay, good point. I can live with an iommu_attach_group_for_dev()
interface, it is still better than making iommu_attach_device() silently
operate on whole groups.

> VFIO needs them because its uAPI is tied, but even so we keep talking
> about ways to narrow the amount of group API it consumes.
> 
> We should not set the recommended/good kAPI based on VFIOs uAPI
> design.

Agree here too. The current way groups are implemented can be turned
into a VFIO specific interface to keep its semantics and kABI. But the
IOMMU core code still needs the concept of alias groups.

Regards,

	Joerg