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

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Message-ID: <20090618182545.GC6838@il.ibm.com>
Date:	Thu, 18 Jun 2009 21:25:45 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	"Yu, Fenghua" <fenghua.yu@...el.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 11:14:51AM -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?

Cheers,
Muli
-- 
Muli Ben-Yehuda | muli@...ibm.com | +972-4-8281080
Manager, Virtualization and Systems Architecture
Master Inventor, IBM Haifa Research Laboratory
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