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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jason Gunthorpe <jgg@nvidia.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>
CC:     David Gibson <david@gibson.dropbear.id.au>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
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        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Alex Williamson (alex.williamson@redhat.com)" 
        <alex.williamson@redhat.com>, Jason Wang <jasowang@redhat.com>,
        Eric Auger <eric.auger@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        "Kirti Wankhede" <kwankhede@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>
Subject: RE: [RFC] /dev/ioasid uAPI proposal
Thread-Topic: [RFC] /dev/ioasid uAPI proposal
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> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Saturday, June 19, 2021 2:31 AM
>=20
> On Fri, Jun 18, 2021 at 07:03:31PM +0200, Jean-Philippe Brucker wrote:
>=20
> > configuration. The Arm SMMUs have a lot of small features that
> > implementations can mix and match and that a user shouldn't have to car=
e
> > about, and there are lots of different implementations by various
> > vendors.
>=20
> 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.

Agree.

>=20
> 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.
>=20
> 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.
>=20

Yes. Except some generic info (e.g. addr_width), most format info can=20
be exposed via a vendor specific data union region. There is no need=20
to define those vendor bits explicitly in the uAPI. Kernel IOMMU driver=20
and vIOMMU emulation logic know how to interpret them.=20

Take VT-d for example, all format/cap info are carried in two registers
(cap_reg and ecap_reg) thus two u64 fields are sufficient...=20

Thanks
Kevin