linux-kernel - Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma

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Message-ID: <20080929093044.GB6931@il.ibm.com>
Date:	Mon, 29 Sep 2008 12:30:44 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
	David Woodhouse <dwmw2@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops

On Sun, Sep 28, 2008 at 09:13:33PM +0200, Joerg Roedel wrote:

> I think we should try to build a paravirtualized IOMMU for KVM
> guests.  It should work this way: We reserve a configurable amount
> of contiguous guest physical memory and map it dma contiguous using
> some kind of hardware IOMMU. This is possible with all hardare
> IOMMUs we have in the field by now, also Calgary and GART. The guest
> does dma_coherent allocations from this memory directly and is done.
> For map_single and map_sg 
> the guest can do bounce buffering. We avoid nearly all pvdma hypercalls
> with this approach, keep guest swapping working and solve also the
> problems with device dma_masks and guest memory that is not contigous on
> the host side.

I'm not sure I follow, but if I understand correctly with this
approach the guest could only DMA into buffers that fall within the
range you allocated for DMA and mapped. Isn't that a pretty nasty
limitation?  The guest would need to bounce-bufer every frame that
happened to not fall inside that range, with the resulting loss of
performance.

Cheers,
Muli
-- 
The First Workshop on I/O Virtualization (WIOV '08)
Dec 2008, San Diego, CA, http://www.usenix.org/wiov08/
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