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[Xen-devel] Re: IOMMU support for xen in BIOS of AMD based HP DL385g7 and kernel 3.0.0-rc7

On Thu, Jul 21, 2011 at 11:53:50AM -0400, M. Schneider wrote:
> Am 20.07.2011 14:45, schrieb Roedel, Joerg:
> > On Sun, Jul 17, 2011 at 06:34:33AM -0400, M. Schneider wrote:
> >    
> >> Please allow me one question about IOMMU support in AMDS's 890FX chipset
> >> and 6176SE CPUs used in HP DL385g7 servers. These chips support IOMMU as
> >> far as I know. BIOS of HP DL385g7 doesn't currently offer an IOMMU flag.
> >>
> >> I have contacted already HP support due to this missing BIOS setting
> >> options but less or more my request has been ignored (no solution and no
> >> plans for solving of this important issue  yet) so I am looking for a
> >> workaround as IOMMU support is required for passing thoruhg of PCI
> >> devices to domU domains (xen 4.0 / 4.1).
> >>      
> > I am sorry but if there is no IVRS table there is not much we can do.
> > The IVRS table contains the information which device uses which
> > request-ids. Without this information the IOMMU can not work reliably.
> >    
> Thank you very much fo your answer Jörg.
> Some BIOSes (like BIOS of HD DL385g7 server with AMD CPUs) don't have 
> the IOMMU option.
> I have to correct my prevoius email as HP DL385g7 servers have the AMD 
> SR5690/SP5100 chipset (supporting IOMMU).
> Is it possible to write a workaround for xen 4.1 or 4.2 to create such 
> IVRS table in code (in case it has static entries for specific server 
> type) and add a boot option like "iommu=force-dl385g7" ?
> Thank you in advance for any hints.

This is basically possible. The Xen code needs to do what the BIOS fails
to do. But for Xen this is a lot more difficult to get right because Xen
can not make the same assumptions about the hardware like a BIOS can.  I
can easily imagine that this will end in a maintenance nighmare if
people start using this. I personally wouldn't do that for Linux, but it
is up to the Xen IOMMU maintainer to decide that for the Xen side. I put
Wei Wang on Cc, he maintains AMD IOMMU for Xen. 



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