Re: [GIT PULL] Intel IOMMU for 4.1

[Alex Williamson <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Mon, 2015-04-27 at 08:48 -0700, Linus Torvalds wrote:
> On Mon, Apr 27, 2015 at 8:39 AM, Alex Williamson
> <alex.williamson-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> >
> > Since this got pulled anyway, do you plan to follow-up with patches to
> > limit the graphics RMRR exception to the known and acceptable uses and
> > document them, or should I send a revert patch?  I don't think what we
> > have here is acceptable going forward or being backported to stable.
> > Thanks,
> 
> Does anybody actually *care* about RMRR?
> 
> The thing is shit. We know it's shit. We already had Linda pipe up to
> tell us that there are bogus RMRR's for ever slot in older machines.
> And we already ignore RMRR for USB controllers because it was just
> useless garbage.
> 
> Is there any reason to *not* just ignore RMRR for all video devices
> like we do now? Seriously?
> 
> I'm *so* not impressed with firmware tables. These things are always
> uniformly wrong. We should strive to generate the information from our
> actual hardware knowledge, with BIOS tables being the absolutely least
> trusted source of information.

It's true that there are RMRRs that are generated simply as placeholders
on some systems and not actually used.  However, it's also the case that
some RMRRs are actively used and used in such a way that if we ignore
them, we end up with corrupted VM memory because the data reporting done
through the RMRR continues whether the IOVA is mapped to the intended
reserved host address or memory backing the VM.  From the kernel
perspective, I don't see how we can tell the difference between
placeholders and active RMRRs, nor do we really have any opportunity to
introspect that this is occurring.  In this data reporting mode, RMRRs
are just a mechanism the platform can use to configure side-band data
reporting between devices, outside of the scope or control of then
kernel.

I certainly don't know of any display devices that do this kind of data
reporting, but I'm not thrilled about assuming they don't, or won't
exist.  Thanks,

Alex