Re: [PATCH v2] Intel IOMMU patch to reprocess RMRR info

[Joerg Roedel <joerg.roedel-5C7GfCeVMHo@public.gmane.org>]

On Fri, Sep 28, 2012 at 06:40:08AM -0600, Alex Williamson wrote:
> On Fri, 2012-09-28 at 11:43 +0200, Joerg Roedel wrote:

> > I don't think so. The concept of RMRR is just not defined well enough
> > (like the concept of unity mappings on the AMD side which is similar to
> >  RMRR). The definition says, that any memory region must be mapped at
> > any time for the device. But that is not true (at least I have no
> > counter-example yet). The right definition would be, that the RMRR
> > regions are only necessary as long as the operating system does not
> > control the particular device. And assigning a device to a guest also
> > counts a 'taking control over the device'.
> 
> I think HP folks would be very unhappy with that definition.  As David
> indicates, that's how things like USB use RMRR, but the actual
> definition in the spec leaves much more room for abuse.  Thanks,

To my experience, for a hardware designer, existing software overrides
any Spec because it is much worse to break existing software than it is
to break a Spec :) So, unless we break existing hardware/firmware, I
still suggest that we use the assumption that OS controlled devices do
not need RMRR/unity-mapped regions anymore.

Is HP doing anything in their firmware which would not work with that?

For the USB controlers, they only generate DMA to the RMRR/unity-mapped
region until the OS takes over control from the firmware. After
the USB driver is initialized the RMRR region should not be necessary
anymore.


	Joerg

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