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