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

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Message-ID: <20080929092549.GA6931@il.ibm.com>
Date:	Mon, 29 Sep 2008 12:25:49 +0300
From:	Muli Ben-Yehuda <muli@...ibm.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	joerg.roedel@....com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
	dwmw2@...radead.org, amit.shah@...ranet.com, mingo@...hat.com
Subject: Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops

On Sun, Sep 28, 2008 at 08:44:24PM +0200, Joerg Roedel wrote:

> Hmm, we should only call find_dma_ops_for_device() the first time a
> dma api call is done (look into get_dma_ops). But I also thought
> about how this lock can be avoided. In the real world it should not
> be necessary because the dma_ops list is initialized before dma api
> calls are done. But since there is now a register function which can
> be called its safer this way. What do you think, are we still safe
> enough without this lock?

We could be, if we add a check to the register function that verifies
it isn't being called after DMAs have started. Something like:

in register:

  if (dma_started)
    yell loudly

before PCI device initialization and after IOMMU initialization:
  dma_started = true

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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