On Mon, 3 Aug 2009, David Woodhouse wrote:
> We are seeing a number of crashes in SMM, when VT-d is enabled while
> 'Legacy USB support' is enabled in various BIOSes.
>
> The BIOS is supposed to indicate which addresses it uses for DMA in a
> special ACPI table ("RMRR"), so that we can punch a hole for it when we
> set up the IOMMU.
>
> The problem is, as usual, that BIOS engineers are totally incompetent.
> They write code which will crash if the DMA goes AWOL, and then they
> either neglect to provide an RMRR table at all, or they put the wrong
> addresses in it. And of course they don't do _any_ QA, since that would
> take too much time away from their crack-smoking habit.
>
> The real fix, of course, is for consumers to refuse to buy motherboards
> which only have closed-source firmware available. If we had _open_
> firmware, bugs like this would be easy to fix.
>
> Since that's something I can only dream about, this patch implements an
> alternative -- ensuring that the USB controllers are handed off from the
> BIOS and quiesced _before_ the IOMMU is initialised. That would have
> been a much better design than this RMRR nonsense in the first place, of
> course. The bootloader has no business doing DMA after the OS has booted
> anyway.
This patch seems reasonable, but you need to run it by the PCI people.
Alan Stern
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