linux-kernel - Re: [PATCH 1/2] IOMMU Identity Mapping Support: iommu_identity

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Message-ID: <20090618185115.GC2500@x200.localdomain>
Date:	Thu, 18 Jun 2009 11:51:15 -0700
From:	Chris Wright <chrisw@...hat.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
Cc:	Chris Wright <chrisw@...hat.com>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"'David Woodhouse'" <dwmw2@...radead.org>,
	"'Linus Torvalds'" <torvalds@...ux-foundation.org>,
	"'Stephen Rothwell'" <sfr@...b.auug.org.au>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>,
	"'Ingo Molnar'" <mingo@...e.hu>,
	"Kay, Allen M" <allen.m.kay@...el.com>,
	"'lkml'" <linux-kernel@...r.kernel.org>,
	"'iommu'" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH 1/2] IOMMU Identity Mapping Support:
	iommu_identity_mapping definition

* Muli Ben-Yehuda (muli@...ibm.com) 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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