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Re: X server in dom0: Bad VBT signature

On Tue, Sep 04, 2012 at 03:32:23PM +0200, Gianluca Guida wrote:
> On Tue, Sep 04, 2012 at 02:29:10PM +0200, Manuel Bouyer wrote:
> > On Tue, Sep 04, 2012 at 02:12:39PM +0200, Gianluca Guida wrote:
> > > This is only partially true. Xen also gives us a memory map through
> > > memory_op hypercall (for dom0, it actually gives us something pretty
> > > similar to the host e820).
> > > With this information, we can map the I/O space and other non-RAM pieces
> > > of memory to identity mapping that access the actual physical memory of
> > > the host (I am talking about dom0 case here, clearly).
> > 
> > And what do you do with ram that Xen did give us at these addresses ?
> > remap it somewhere else ?
> My memory isn't helping me here, as the last time I looked at this code
> in the Linux kernel was in 2009, and then I've stopped working on Xen
> for almost two years, but:
> IIRC Linux's dom0 kernel gives back the memory to Xen. I will look at

Ha yes, that's another possibility.

> the details later. It should probably be possible to remap the memory in
> some other part, though it might be messy as we'll have to carefully
> handle the various holes in the e820 map.
> > The identity mapping isn't even needed, if the address is not in a
> > ram segment it's a machine address.
> > 
> > But such a hack doesn't solve the underlying problem which is,
> > basically, that under Xen, physical and machine addresses are
> > different address spaces.
> > 
> I have trouble following you here. We seem to see the problem from a very
> different points of view. And I fail to understand why this should be a
> hack.

The real problem is that X, unlike in-kernel drivers, can't express
that it wants to access device memory space and not plain RAM. Splitting
the /dev/mem space in RAM and non-RAM segments is only a workaround for
that. This workaround works for x86, but one could immagine a hardware
where this woudln't work.

Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference

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