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Re: [Xen-devel] [PATCH 2/2] xen/dom0: Add a dom0-iommu=none option

On Fri, Dec 21, 2018 at 01:01:22PM +0000, Andrew Cooper wrote:
> On 21/12/2018 12:44, Roger Pau Monné wrote:
> > On Thu, Dec 20, 2018 at 11:40:52PM +0000, Andrew Cooper wrote:
> >> For development purposes, it is very convenient to boot Xen as a PVH guest,
> >> with an XTF PV or PVH "dom0".  The edit-compile-go cycle is a matter of
> >> seconds, and you can resonably insert printk() debugging in places which 
> >> which
> >> would be completely infeasible when booting fully-fledged guests.
> >>
> >> However, the PVH dom0 path insists on having a working IOMMU, which doesn't
> >> exist when virtualised as a PVH guest, and isn't necessary for XTF anyway.
> >>
> >> Introduce a developer mode to skip the IOMMU requirement.
> > This looks very similar to the current 'passthrough' option, maybe it
> > would be enough to allow PVH dom0 to use the passthrough option
> > provided a warning is added to
> > arch_iommu_check_autotranslated_hwdom?
> I considered that, but "dom0-iommu=passthrough" isn't an accurate
> description of what is going on.  Frankly, its not correct for PV either.

And what about turning dom0-iommu into a boolean option itself, so
that you can do "dom0-iommu=false"?

I think that's more similar to other Xen command line options that
have a boolean value and/or a list of sub-options.

> TBH, dom0-iommu=none is better for both.  How about I introduce that as
> the new option, and leave passthrough as a legacy alias?

That sounds good, I would like to avoid (if possible) the
proliferation of new iommu_hwdom_* variables, because we already have
a bunch and the functionality introduced by the 'none' option looks
very similar to what 'passthrough' aims to achieve.

Thanks, Roger.

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