linux-kernel - Re: [PATCH 0/2] x86: per-device dma_mapping

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Message-ID: <20080515213946.GE10434@il.ibm.com>
Date:	Fri, 16 May 2008 00:39:46 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	alexisb@...ibm.com
Subject: Re: [PATCH 0/2] x86: per-device dma_mapping_ops

On Thu, May 15, 2008 at 11:03:40PM +0200, Andi Kleen wrote:
> On Thu, May 15, 2008 at 06:26:31PM +0300, Muli Ben-Yehuda wrote:
> > On Thu, May 15, 2008 at 06:41:26PM +0900, FUJITA Tomonori wrote:
> > 
> > > I thought that KVM people want to do it per device (in the first
> > > case). So with my patchse, they can replace the dma_ops pointer
> > > in dev_archdata with what they want.
> > 
> > That's my understanding too. We use stackable ops as a poor man's
> > replacement for per-device ops (depending on what kind of device
> > it is, call the original ops or our pvdma ops).
> 
> But in the KVM case you still need to support the underlying ops
> too, e.g. in case of bouncing through swiotlb needed

Good point, although "secondary" DMA-ops should only be needed in rare
cases (the only one I can think of is bouncing through swiotlb for a
pass-through device which has a limited DMA mask), whereas per-device
ops are needed for every pass-through device.

Cheers,
Muli
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