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

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Message-ID: <A6AD88C3F2289247BE726C37303E1EB8B0044EC0@orsmsx505.amr.corp.intel.com>
Date:	Thu, 18 Jun 2009 11:14:51 -0700
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	'Muli Ben-Yehuda' <muli@...ibm.com>
CC:	'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

>
>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.

By default, identity mapping is turned off. "iommu=identity" can turn it on

Thanks.

-Fenghua
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