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

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Message-Id: <20080928231211X.fujita.tomonori@lab.ntt.co.jp>
Date:	Sun, 28 Sep 2008 23:21:23 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	joerg.roedel@....com
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	iommu@...ts.linux-foundation.org, dwmw2@...radead.org,
	muli@...ibm.com, amit.shah@...ranet.com, mingo@...hat.com,
	fujita.tomonori@....ntt.co.jp
Subject: Re: [PATCH 9/9] x86/iommu: use dma_ops_list in get_dma_ops

On Mon, 22 Sep 2008 20:21:21 +0200
Joerg Roedel <joerg.roedel@....com> wrote:

> This patch enables stackable dma_ops on x86. To do this, it also enables
> the per-device dma_ops on i386.
> 
> Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> ---
>  arch/x86/kernel/pci-dma.c     |   26 ++++++++++++++++++++++++++
>  include/asm-x86/device.h      |    6 +++---
>  include/asm-x86/dma-mapping.h |   14 +++++++-------
>  3 files changed, 36 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index b990fb6..2e517c2 100644
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -82,6 +82,32 @@ void x86_register_dma_ops(struct dma_mapping_ops *ops,
>  	write_unlock_irqrestore(&dma_ops_list_lock, flags);
>  }
>  
> +struct dma_mapping_ops *find_dma_ops_for_device(struct device *dev)
> +{
> +	int i;
> +	unsigned long flags;
> +	struct dma_mapping_ops *entry, *ops = NULL;
> +
> +	read_lock_irqsave(&dma_ops_list_lock, flags);
> +
> +	for (i = 0; i < DMA_OPS_TYPE_MAX; ++i)
> +		list_for_each_entry(entry, &dma_ops_list[i], list) {
> +			if (!entry->device_supported)
> +				continue;
> +			if (entry->device_supported(dev)) {
> +				ops = entry;
> +				goto out;
> +			}
> +		}
> +out:
> +	read_unlock_irqrestore(&dma_ops_list_lock, flags);

Hmm, every time we call dma_sg/map_single, we call
read_lock_irqsave(&dma_ops_list_lock, flags). It's likely that we see
notable performance drop?
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