config changes

[Gerd Hoffmann]

Dave Jones wrote:
> On Tue, Jan 13, 2009 at 09:55:27PM +0100, Gerd Hoffmann wrote:
>  > Bill Nottingham wrote:
>  > > Here's some proposed config changes.
>  > 
>  > Jumping on the wagon ;)
>  > 
>  > Can we enable CONFIG_DMAR please?  This turns on the IOMMU on intel
>  > boxes, using VT-d.  Called "DMA Remapping" in intel speak, this is where
>  > the config option name comes from.  Advantages:
>  > 
>  >   (1) 32bit PCI devices can DMA to memory above 4G, thus the need for
>  >       swiotlb (i.e. bounce buffers) is gone.
>  >   (2) It allows kvm to pass through PCI devices to guests securely.
>  
> The last time we tried this, it blew up a lot due to broken BIOSes.

Do you have bug numbers at hand?
Searching for CONFIG_DMAR gives me "Zarro Boogs found".

> Maybe it's been improved enough to tolerate them, so we can probably
> give it a spin in rawhide for a while to see what happens.

Well, it works for me.  Single test box only though.  As the machine in
question actually has memory above 4G it is a nice speedup for kernel
builds (sys time going down from ~15min to ~5min).

cheers,
  Gerd