linux-kernel - Re: [Intel IOMMU][patch 4/8] Supporting Zero Length Reads in Intel IOMMU.

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Date:	Tue, 24 Apr 2007 14:00:55 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ak@...e.de
Cc:	ashok.raj@...el.com, linux-kernel@...r.kernel.org, akpm@...l.org,
	gregkh@...e.de, muli@...ibm.com, asit.k.mallick@...el.com,
	suresh.b.siddha@...el.com, anil.s.keshavamurthy@...el.com,
	arjan@...ux.intel.com, shaohua.li@...el.com
Subject: Re: [Intel IOMMU][patch 4/8] Supporting Zero Length Reads in Intel
 IOMMU.

From: Andi Kleen <ak@...e.de>
Date: Tue, 24 Apr 2007 21:28:11 +0200

> On Tuesday 24 April 2007 08:03:03 Ashok Raj wrote:
> > PCI specs permit zero length reads (ZLR) even if the mapping for that region 
> > is write only. Support for this feature is indicated by the presence of a bit 
> > in the DMAR capability. If a particular DMAR does not support this capability
> > we map write-only regions as read-write.
> > 
> > This option can also provides a workaround for some drivers that request
> > a write-only mapping when they really should request a read-write.
> > (We ran into one such case in eepro100.c in handling rx_ring_dma)
> 
> Better just fix the drivers instead of adding such hacks

I totally agree.

It is a feature when we have a device which can enforce such
permissions correctly and thus trap such bad uses.  If you
paper around it, these things will never get fixed.
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