[Xen-devel] crash tool - problem with new Xen linear virtual mapped sparse p2m list

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Hi all,

Some time ago Linux kernel commit 054954eb051f35e74b75a566a96fe756015352c8
(xen: switch to linear virtual mapped sparse p2m list) introduced linear
virtual mapped sparse p2m list. It fixed some issues, however, it broke
crash tool too. I tried to fix this issue but the problem looks more
difficult than I expected.

Let's focus on "crash vmcore vmlinux". vmcore file was generated from dom0.
"crash vmcore xen-syms" works without any issue.

At first sight problem looks simple. Just add a function which reads p2m list
from vmcore and voila. I have done it. Then another issue arisen.

Please take a look at following backtrace:

#24426 0x000000000048b0f6 in readmem (addr=18446683600570023936, memtype=1, 
buffer=0x3c0a060, size=4096,
    type=0x900b2f "xen_p2m_addr page", error_handle=2) at memory.c:2157
#24427 0x000000000050f746 in __xen_pvops_m2p_vma (machine=5323599872, 
mfn=1299707) at kernel.c:9050
#24428 0x000000000050edb7 in __xen_m2p (machine=5323599872, mfn=1299707) at 
kernel.c:8867
#24429 0x000000000050e948 in xen_m2p (machine=5323599872) at kernel.c:8796
#24430 0x0000000000528fca in x86_64_kvtop_xen_wpt (tc=0x0, 
kvaddr=18446683600570023936, paddr=0x7fff51c7c100, verbose=0)
    at x86_64.c:1997
#24431 0x0000000000528890 in x86_64_kvtop (tc=0x0, kvaddr=18446683600570023936, 
paddr=0x7fff51c7c100, verbose=0)
    at x86_64.c:1887
#24432 0x000000000048d708 in kvtop (tc=0x0, kvaddr=18446683600570023936, 
paddr=0x7fff51c7c100, verbose=0) at memory.c:2900
#24433 0x000000000048b0f6 in readmem (addr=18446683600570023936, memtype=1, 
buffer=0x3c0a060, size=4096,
    type=0x900b2f "xen_p2m_addr page", error_handle=2) at memory.c:2157
#24434 0x000000000050f746 in __xen_pvops_m2p_vma (machine=5323599872, 
mfn=1299707) at kernel.c:9050
#24435 0x000000000050edb7 in __xen_m2p (machine=5323599872, mfn=1299707) at 
kernel.c:8867
#24436 0x000000000050e948 in xen_m2p (machine=5323599872) at kernel.c:8796
#24437 0x0000000000528fca in x86_64_kvtop_xen_wpt (tc=0x0, 
kvaddr=18446683600570023936, paddr=0x7fff51c7ca60, verbose=0)
    at x86_64.c:1997
#24438 0x0000000000528890 in x86_64_kvtop (tc=0x0, kvaddr=18446683600570023936, 
paddr=0x7fff51c7ca60, verbose=0)
    at x86_64.c:1887
#24439 0x000000000048d708 in kvtop (tc=0x0, kvaddr=18446683600570023936, 
paddr=0x7fff51c7ca60, verbose=0) at memory.c:2900
#24440 0x000000000048b0f6 in readmem (addr=18446683600570023936, memtype=1, 
buffer=0x3c0a060, size=4096,
    type=0x900b2f "xen_p2m_addr page", error_handle=2) at memory.c:2157
#24441 0x000000000050f746 in __xen_pvops_m2p_vma (machine=6364917760, 
mfn=1553935) at kernel.c:9050
#24442 0x000000000050edb7 in __xen_m2p (machine=6364917760, mfn=1553935) at 
kernel.c:8867
#24443 0x000000000050e948 in xen_m2p (machine=6364917760) at kernel.c:8796
#24444 0x0000000000528fca in x86_64_kvtop_xen_wpt (tc=0x0, 
kvaddr=18446744072099176512, paddr=0x7fff51c7d3c0, verbose=0)
    at x86_64.c:1997
#24445 0x0000000000528890 in x86_64_kvtop (tc=0x0, kvaddr=18446744072099176512, 
paddr=0x7fff51c7d3c0, verbose=0)
    at x86_64.c:1887
#24446 0x000000000048d708 in kvtop (tc=0x0, kvaddr=18446744072099176512, 
paddr=0x7fff51c7d3c0, verbose=0) at memory.c:2900
#24447 0x000000000048b0f6 in readmem (addr=18446744072099176512, memtype=1, 
buffer=0xfbb500, size=768,
    type=0x8fd772 "module struct", error_handle=6) at memory.c:2157
#24448 0x00000000004fb0ab in module_init () at kernel.c:3355

As you can see module_init() calls readmem() which attempts to read virtual 
address
which lies outside of kernel text mapping (0xffffffff80000000 - 
0xffffffffa0000000).
In this case addr=18446744072099176512 == 0xffffffffa003a040 which is known as 
module
mapping space. readmem() needs physical address, so, it calls kvtop() then 
kvtop()
calls x86_64_kvtop(). x86_64_kvtop() is not able to easily calculate, using 
simple
arithmetic like in case of kernel text mapping space, physical address from 
virtual
address. Hence it calls x86_64_kvtop_xen_wpt() to calculate it by traversing 
page
tables. x86_64_kvtop_xen_wpt() needs to do some m2p translation so it calls 
xen_m2p()
which calls __xen_m2p() and finally it calls __xen_pvops_m2p_vma() (my function 
which
tries to read linear virtual mapped sparse p2m list). Then 
__xen_pvops_m2p_vma() calls
readmem() which tries to read addr=18446683600570023936 == 0xffffc90000000000 
which
is VMA used for m2p list. Well, once again physical address must be calculated 
by
traversing page tables. However, this requires access to m2p list which leads to
another readmem() call. Starting from here we are in the loop. After thousands 
of
repetitions crash dies due to stack overflow. Not nice... :-(((

Do we have any viable fix for this issue? I considered a few but I have not 
found prefect one.

1) In theory we can use p2m tree to solve that problem because it is available 
in parallel
   with VMA mapped p2m right now. However, this is temporary solution and it 
will be phased
   out sooner or later. We need long term solution.

2) As I saw crash tool creates xkd->p2m_mfn_frame_list from Xen 
crash_xen_info_t.dom0_pfn_to_mfn_frame_list_list
   in dom0 case. Potentially this would solve the problem for dom0 crash dumps 
but it does
   not solve the problem for PV guests in general. We need something which is 
more generic.

3) The best thing which I was able to think of is to put list of PFNs 
containing p2m list
   somewhere in ELF notes. Then readmem() called from __xen_pvops_m2p_vma() 
could use this
   physical addresses calculated from PFNs somehow. However, this is also not 
perfect because
   it requires changes in kernel, and/or xl and crash. Additionally, this 
solution increases
   ELF notes. Every 1 GiB of memory will add 2 MiB of PFNs to ELF note. 
Probably there is
   a chance that we can employ a compression method to reduce ELF note size 
but...

So, as you can see there is not perfect solution for this issue.
Hmmm... Should we mix solution #2 and #3? Is it better thing?

Daniel

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