Re: [PATCH] intel-iommu: respect iommu=pt

["Woodhouse, David" <david.woodhouse-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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On Tue, 2015-11-24 at 13:53 -0800, Shaohua Li wrote:
> Currently if a device has rmrr or the device is a pci device,
> passthrough is disabled even with iommu=pt. The worry is moving such
> devices between domains don't work. But some users don't do the domain
> reassignment at all, disabling passthough punish everybody. And iommu=pt
> is a boot option, user knows the risk. So intead of failing the
> passthough, just print a warning and continue the passthough.

We tend to suggest that iommu=pt gives you performance for the *decent*
devices, while dubious devices still get translated. So your crappy 32-
bit devices still get translated. And anything unfortunate enough to
have an RMRR can *still* get translated.

I'm reluctant to change that. If it hurts you, kick your system vendor
until they stop doing stupid things with RMRRs.

I think we also need to revisit the whole 'iommu=pt' thing anyway and
define the semantics we expect across archiectures — given that SPARC
and POWER are doing passthrough for 'decent' devices by *default*. So
let's not tweak it right now.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org                              Intel Corporation


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