Messages in this thread | | | Date | Tue, 28 Apr 2009 17:05:31 +0200 | From | Joerg Roedel <> | Subject | IOMMU and graphics cards |
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Hi David,
as I have seen the VT-d code implements a workaround for broken graphics card drivers. What it does, when enabled, is giving each grahics device direct access to all physical memory. I really don't like to implement this but a similar workaround for the AMD IOMMU seems to be necessary. The biggest problem here is that this kind of workaround disables device isolation for graphics cards. Device isolation is the main reason for using an IOMMU in an unvirtualized environment. So if this workaround is enabled it is as good as disable the IOMMU at all. Thats why I think a kernel compile option for this workaround is not sufficient. Distributors will probably enable this in their kernels which also disables device isolation even if the user don't want to use these broken drivers. I think we should change that and provide a better way which allows to enable this workaround only if it is required (and it should be transparent to the user which IOMMU is built into the system). We have several options to do this:
* Implement a kernel command line option to enable/disable the workaround (what should be default?) * Use the IOMMU-API and write a kernel module which creates a direct mapped protection domain and assigns the graphics cards to it (need to be done carefully to not break graphics drivers which do everything right and use the DMA-API) * Any other great idea?
So what do you (and all the others reading this :-) think? I would prefer the way of implementing a module but there may also be reasons against this. I would like this to be disussed before I implement the workaround for the AMD IOMMU.
Thanks,
Joerg
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