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Re: [PATCH v2 8/8] docs: Add '-device virtio-iommu' entry


From: Jean-Philippe Brucker
Subject: Re: [PATCH v2 8/8] docs: Add '-device virtio-iommu' entry
Date: Wed, 8 Sep 2021 11:14:20 +0100

On Mon, Sep 06, 2021 at 03:45:28PM +0100, Daniel P. Berrangé wrote:
> On Fri, Sep 03, 2021 at 04:32:09PM +0200, Jean-Philippe Brucker wrote:
> > Document the virtio-iommu device for qemu-system-x86_64. In particular
> > note the lack of interrupt remapping, which may be an important
> > limitation on x86.
> > 
> > Suggested-by: Eric Auger <eric.auger@redhat.com>
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > ---
> >  qemu-options.hx | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 83aa59a920..9a1906a748 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -976,6 +976,9 @@ SRST
> >      Please also refer to the wiki page for general scenarios of VT-d
> >      emulation in QEMU: https://wiki.qemu.org/Features/VT-d.
> >  
> > +``-device virtio-iommu``
> > +    Enable a paravirtual IOMMU, that manages DMA isolation and remapping
> > +    for all PCI devices, but does not support interrupt remapping.
> 
> It would be desirable to document why this is better/worse/equiv to
> the intel-iommu device documented just before, so that people have a
> better idea of which they should be trying to use.
> 
> I'm going to assume intel-iommu is more likely to "just work" out of
> the box since it models real hardware that OS are likely to already
> support ?  Is that right though ?

That's right, and there isn't much point in using virtio-iommu on q35 at
the moment, we're still laying the foundation. It only implements caching
mode for now so it complements the vSMMU on arm which doesn't have that,
but on x86 QEMU it doesn't add anything useful. There are extensions to
get better performance for different type of endpoint, and other features,
but those are all work in progress.

I thinks I'll just drop the documentation patch until the device is more
mature and I have something useful to write here. The other IOMMUs are not
described either at the moment. x86 users will just instantiate the
intel-iommu as usual if they need an IOMMU.

Thanks,
Jean



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