Re: [PATCH 1/2] IOMMU Identity Mapping Support:iommu_identity_mapping definition

[Chris Wright]

* Muli Ben-Yehuda (muli@xxxxxxxxxx) wrote:
> On Thu, Jun 18, 2009 at 11:31:21AM -0700, Chris Wright wrote:
> 
> > The 1-1 mapping is for all the host devices _not_ assigned to
> > guests.  To eliminate the i/o overhead imposed on all guests not
> > using an assigned device or from i/o from host.
> >
> > It's just the same as VT-d PassThrough mode for hardware that
> > doesn't support it.
> 
> Ok, that makes sense. Thanks, Chris. However, that doesn't appear to
> be what the patch does---unless I'm misreading, if
> iommu_identity_mapping is set, *all* devices get identity

Correct (and same as pt mode).  Of course, a subsequent device assignement
will set that mapping to gpa<->hpa, so you'll get proper isolation for
the assigned device.  Essentially it's saying the only reason you cared
about the IOMMU was for device assignment.

> mapping. Instead of a global command line option, we need to provide a
> way to enable/disable pt or identity mapping (I agree that the user
> shouldn't know or care which is used, the kernel should pick the best
> one automatically) on a per BDF basis.

Yeah, that's a possible enhancement.  Although once you've disabled one
device's isolation you've created a hole...

thanks,
-chris
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