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

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Date:   Fri, 18 Jun 2021 15:30:54 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     David Gibson <david@...son.dropbear.id.au>,
        "Tian, Kevin" <kevin.tian@...el.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>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Fri, Jun 18, 2021 at 07:03:31PM +0200, Jean-Philippe Brucker wrote:

> configuration. The Arm SMMUs have a lot of small features that
> implementations can mix and match and that a user shouldn't have to care
> about, and there are lots of different implementations by various
> vendors.

This is really something to think about carefully in this RFC - I do
have a guess that a 'HW specific' channel is going to be useful here.

If the goal is for qemu to provide all these fiddly things and they
cannot be SW emulated, then direct access to the fiddly HW native
stuff is possibly necessary.

We've kind of seen this mistake in DRM and RDMA
historically. Attempting to generalize too early, or generalize
something that is really a one off. Better for everyone to just keep
it as a one off.

Jason