Re: p2m stuff and crash tool

[Daniel Kiper]

On Wed, Feb 17, 2016 at 03:27:01PM +0100, Juergen Gross wrote:
> On 17/02/16 14:59, Daniel Kiper wrote:
> > On Tue, Feb 16, 2016 at 01:55:33PM +0100, Juergen Gross wrote:
> >> 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.
> >
> > That make sense.
> >
> >> 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()).
> >
> > However, I have memory hotplug disabled and kernel set xen_max_p2m_pfn
> > to 0x80000 (2 Gib) even if dom0 memory was set to 1 GiB. Hmmm... Why?
> > I suppose that if xen_max_p2m_pfn == max_pfn then everything should work.
> > Am I missing something?
>
> The virtual p2m list's size is aligned to PMD_SIZE (2 MB). For 1 GB dom0
> memory max_pfn will be a little bit above 0x40000 due to the BIOS
> area resulting in a 4 MB p2m list. And xen_max_p2m_pfn is reflecting
> this size. You could reduce it to max_pfn without any problem, as long
> as memory hotplug is disabled. At least I think so.

To be precise PMD_SIZE * PMDS_PER_MID_PAGE, so, it equals to 0x80000 in
this case. Why we need so huge alignment? Could not we use smaller one,
e.g. PAGE_SIZE?

Daniel