netdev - Re: network devices don't handle pci_dma_mapping

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Date:	Wed, 06 Dec 2006 23:24:59 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	amitkale@...syssoft.com
Cc:	shemminger@...l.org, muli@...ibm.com, jeff@...zik.org,
	amitkale@...xen.com, netdev@...r.kernel.org, brazilnut@...ibm.com,
	netxenproj@...syssoft.com, rob@...xen.com, romieu@...zoreil.com,
	sanjeev@...xen.com, wendyx@...ibm.com
Subject: Re: network devices don't handle pci_dma_mapping_error()'s

From: "Amit S. Kale" <amitkale@...syssoft.com>
Date: Thu, 7 Dec 2006 11:55:22 +0530

> We can let a driver handle dma mapping errors using these->
> 
> 1.Reduce the size of a receive ring. This will free some possibly remapped 
> memory, reducing pressure on iommu. We also need to printk a message so that 
> a user knows the reason why receive ring was shrunk. Growing it when iommu 
> pressure goes down will result in a ping-pong.
> 2. Force processing of receive and transmit ring. This will ensure that the 
> buffers processed by hardware are freed, reducing iommu pressure.
> 
> 3. If we need to do (1) and (2) a predefined number of times (say 20), stop 
> the queue. Stopping the queue in general will cause a ping-pong, so it should 
> be avoided as far as possible.

This scheme assumes the networking card is the culprit.  In many
workloads it will not be and these efforts will be in vain and perhaps
even make the situation worse.  There's not reason to run the RX and
TX queues, and even shrink them, when the FC controller has most of
the IOMMU entires tied up.

That's why users needs to queue up and get feedback when IOMMU space
is made available.
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