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

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Message-ID: <YKtcFdW9U2+iW4cM@yekko>
Date:   Mon, 24 May 2021 17:56:05 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        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>,
        "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>,
        "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>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

On Thu, May 13, 2021 at 10:50:30AM -0300, Jason Gunthorpe wrote:
> On Thu, May 13, 2021 at 04:07:07PM +1000, David Gibson wrote:
> > On Wed, May 05, 2021 at 01:39:02PM -0300, Jason Gunthorpe wrote:
> > > On Wed, May 05, 2021 at 02:28:53PM +1000, Alexey Kardashevskiy wrote:
> > > 
> > > > This is a good feature in general when let's say there is a linux supported
> > > > device which has a proprietary device firmware update tool which only exists
> > > > as an x86 binary and your hardware is not x86 - running qemu + vfio in full
> > > > emulation would provide a way to run the tool to update a physical device.
> > > 
> > > That specific use case doesn't really need a vIOMMU though, does it?
> > 
> > Possibly not, but the mechanics needed to do vIOMMU on different host
> > IOMMU aren't really different from what you need for a no-vIOMMU
> > guest.  
> 
> For very simple vIOMMUs this might be true, but this new features of nesting
> PASID, migration, etc, etc all make the vIOMMU complicated and
> emuluating it completely alot harder.

Well, sure, emulating a complex vIOMMU is complex. But "very simple
vIOMMUs" covers the vast majority of currently deployed hardware, and
several are already emulated by qemu.

> Stuffing a vfio-pci into a guest and creating a physical map using a
> single IOASID is comparably trivial.

Note that for PAPR (POWER guest) systems this is not an option: the
PAPR platform *always* has a vIOMMU.

-- 
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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