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

From: David Miller
Date: Tue Apr 24 2007 - 17:01:07 EST

From: Andi Kleen <ak@xxxxxxx>
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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