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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.06.12 at 20:35, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> 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?

There can be multi-terabyte-sized non-RAM regions on systems
with discontiguous RAM.

Jan


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