Re: [Crash-utility] p2m stuff and crash tool

Hi Daniel, On 16/02/16 12:35, Daniel Kiper wrote: > Hey Juergen,

> After that I decided to take a look at Linux kernel upstream. I saw > that xen_max_p2m_pfn in xen_build_mfn_list_list() is equal to "the > end of last usable machine memory region available for a given > dom0_mem argument + something", e.g. > > For dom0_mem=1g,max:1g: > > (XEN) Xen-e820 RAM map: > (XEN) 0000000000000000 - 000000000009fc00 (usable) > (XEN) 000000000009fc00 - 00000000000a0000 (reserved) > (XEN) 00000000000f0000 - 0000000000100000 (reserved) > (XEN) 0000000000100000 - 000000007ffdf000 (usable) <--- HERE > (XEN) 000000007ffdf000 - 0000000080000000 (reserved) > (XEN) 00000000b0000000 - 00000000c0000000 (reserved) > (XEN) 00000000feffc000 - 00000000ff000000 (reserved) > (XEN) 00000000fffc0000 - 0000000100000000 (reserved) > (XEN) 0000000100000000 - 0000000180000000 (usable) > > Hence xen_max_p2m_pfn == 0x80000 > > Later I reviewed most of your p2m related commits and I realized > that you played whack-a-mole game with p2m bugs. Sadly, I was not > able to identify exactly one (or more) commit which would fix the > same issue (well, there are some which fixes similar stuff but not > the same one described above). So, if you explain to me why > xen_max_p2m_pfn is set to that value and does not e.g. max_pfn then > it will be much easier for me to write proper fix and maybe fix > the same issue in upstream kernel if it is needed (well, crash > tool does not work with new p2m layout so first of all I must fix it; > I hope that you will help me to that sooner or later). The reason for setting xen_max_p2m_pfn to nr_pages initially is it's usage in __pfn_to_mfn(): this must work with the initial p2m list supplied by the hypervisor which just has only nr_pages entries.

Later it is updated to the number of entries the linear p2m list is able to hold. This size has to include possible hotplugged memory in prder to be able to make use of that memory later (remember: the p2m list's size is limited by the virtual space allocated for it via xen_vmalloc_p2m_tree()).