[Xen-devel] Re: HYBRID: gnttab_map() to map shared frames..

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2011-10-18 at 20:17 +0100, Mukesh Rathor wrote:
> On Tue, 18 Oct 2011 09:13:02 +0100
> Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> ... 
> > Could GNTTABOP_setup_table return GFNs from the very top of the GFN
> > space? Perhaps even above what would be architecturally valid,
> > although perhaps that is incompatible with HAP. Anything above
> > max_pfn would seem to be valid for the hypervisor to place "magic"
> > mappings in?
> 
> Well, what's confusing me for this is that there are few max 
> pfn's inside the guest! The nr_pages being one for example, then 
> there's max_pfn in shared_info.arch. I'm not sure if these two are in 
> sync with max_pfn in mm/bootmem.c. Then, we would need to adjust EPT to add 
> these pfns there. So that may put a damper on this.

What about using PFNs from right at the top, i.e. right up near
0xffffffff? I don't think there's any particular reason these special
PFNs need to be contiguous with the "regular" ones. This would limit the
total amount of RAM you could give a hybrid guest, but not by much.

> > Another approach might be to take advantage of the fact that a modern
> > PV kernel will call XENMEM_memory_map to obtain an e820 describing its
> > pseudo-physical address space. You could arrange for there to be some
> > reserved space returned in that.
> 
> I saw e820 code last night, and wondered if this might be the best option.
> I am glad you mentioned it. I am not much familiar with e820 format, so 
> studying it right now.
> 
> thanks,
> Mukesh
> 



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