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

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Message-ID: <20090618183121.GB2500@x200.localdomain>
Date:	Thu, 18 Jun 2009 11:31:21 -0700
From:	Chris Wright <chrisw@...hat.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
Cc:	"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>,
	"'Christopher Wright'" <chrisw@...hat.com>,
	"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 11:14:51AM -0700, Yu, Fenghua wrote:
> > >
> > >On Thu, Jun 18, 2009 at 11:05:14AM -0700, Fenghua Yu wrote:
> > >
> > >> IOMMU Identity Mapping Support: iommu_identity_mapping definition
> > >>
> > >> Identity mapping for IOMMU defines a single domain to 1:1 map all
> > >> pci devices to all usable memory.
> > >
> > >Why use VT-d at all in this case? Do you have a use-case in mind?
> > 
> > Some users want to use VT-d in KVM but are concerned of DMA
> > remapping performance. They can use identity mapping and still have
> > KVM on VT-d. They can also use pass through patch (sent out before)
> > if hardware supports pass through.
> 
> Sorry, I must be missing something. For the normal device assignment
> case, we want the IOMMU page tables to have gpa->hpa mappings rather
> than the 1-1 identity mapping. How do you envision the 1-1 mapping
> being used in the device assignment case?

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.

thanks,
-chris
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