Re: [Xen-devel] [HYBRID] : mapping IO mems in the EPT

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jun 15, 2012 at 12:02:19PM +0100, Stefano Stabellini wrote:
> On Fri, 15 Jun 2012, Mukesh Rathor wrote:
> > Hi guys,
> > 
> > During my refresh to latest linux, I noticed, direct mapping of all
> > non-RAM pages in xen_set_identity_and_release(). I currently don't map
> > all at front, but as needed looking at the PAGE_IO bit in the pte. One

PV doesn't look at that all the time either. The P2M tree code
has a couple of leafs, that if they have IDENTITY_FRAME_BIT set it will
automatically stick _PAGE_IOMAP on the PTE.

> > result of that is minor change to common code macro:
> > 
> >         __set_fixmap(idx, phys, PAGE_KERNEL_NOCACHE) to
> > to        __set_fixmap(idx, phys, PAGE_KERNEL_IO_NOCACHE)

I am really wafling on that. Jeremy posted a patch some time ago
to x86 folks that would do something similar (I can't remember the
details), but hpa said - why don't you just consult the E820.

That is where the IDENTITY_FRAME_BIT thing in the P2M tree came
about. It could probably be implemented for your cases using ranges.

Similary to how Xen permits/disallows certain IO regions to be touched.
Would something like that potentially allow you do something like this:

xen_hybrid_pte()

phys_addr_t phys = pte.pte & PTE_PFN_MASK

if (phys .. within ranges)
        pte |= _PAGE_IOMAP;
return pte;

> >
> > 
> > To avoid this change, and keep all my changes limited to xen files only,
> > I thought I could just map the entire non-ram pages up front too. But
> > I am concerned the EPT may grow too large? Specially, when we get to 
> > *really* large NUMA boxes. What do you guys think? Should I worry about
> > it?

Would NUMA boxes have more than 1GB of E820 non-RAM regions? I can see them
having gobs of RAM regions, but non-RAM regions?

> 
> I would map them all up front and worry about it later.
> 
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