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

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Message-ID: <20080927001321.GI9118@il.ibm.com>
Date:	Sat, 27 Sep 2008 03:13:21 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	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 Fri, Sep 26, 2008 at 02:32:43PM +0200, Joerg Roedel wrote:

> Ok, the allocation only matters for dma_alloc_coherent. Fujita
> introduced a generic software-based dma_alloc_coherent recently
> which you can use for that. I think implementing PVDMA into an own
> dma_ops backend and multiplex it using my patches introduces less
> overhead than an additional layer over the current dma_ops
> implementation.

I'm not sure what you have in mind, but I agree with Amit that
conceptually pvdma should be called after the guest's "native" dma_ops
have done their thing. This is not just for nommu, consider a guest
that is using an (emulated) hardware IOMMU, or that wants to use
swiotlb. We can't replicate their functionality in the pv_dma_ops
layer, we have to let them run first and then pass deal with whatever
we get back.

> Another two questions to your approach: What happens if a
> dma_alloc_coherent allocation crosses page boundarys and the gpa's
> are not contiguous in host memory? How will dma masks be handled?

That's a very good question. The host will need to be aware of a
device's DMA capabilities in order to return I/O addresses (which
could be hpa's if you don't have an IOMMU) that satisfy them. That's
quite a pain.

Cheers,
Muli
-- 
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Dec 2008, San Diego, CA, http://www.usenix.org/wiov08/
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