[Crash-utility] increase __PHYSICAL_MASK_SHIFT_XEN?
Hi,
I've just come across a situation where crash failed to open a
dump generated by a 4.12 XEN PV dom0 kernel, terminating with
this message:
crash: read error: physical address: ffffffffffffffff type: "pud page"
The problem is a failed machine-to-physical translation.
xen_m2p() returns an error (-1UL) and x86_64_pud_offset() than
uses that value as a physical address.
I debugged the problem by running crash inside gdb. The backtrace was:
#0 xen_m2p (machine=973135872) at kernel.c:9714
#1 0x000000000053ed4a in x86_64_pud_offset (pgd_pte=3299508019303,
vaddr=18446744072642223552, verbose=0, IS_XEN=1)
at x86_64.c:1889
#2 0x0000000000540c8a in x86_64_kvtop_xen_wpt (tc=0x0, kvaddr=18446744072642223552,
paddr=0x7fffffffdb30, verbose=0)
at x86_64.c:2523
#3 0x00000000005407d0 in x86_64_kvtop (tc=0x0, kvaddr=18446744072642223552,
paddr=0x7fffffffdb30, verbose=0)
at x86_64.c:2413
#4 0x0000000000491a97 in kvtop (tc=0x0, kvaddr=18446744072642223552,
paddr=0x7fffffffdb30, verbose=0) at memory.c:3062
#5 0x000000000048f3f0 in readmem (addr=18446744072642223552, memtype=1, buffer=0xba63a0
<shared_bufs>, size=832,
type=0x92e1f2 "module struct", error_handle=6) at memory.c:2314
#6 0x00000000005071e2 in module_init () at kernel.c:3699
....
I tracked the problem to a wrong value of
__PHYSICAL_MASK_SHIFT_XEN. The current value of 40 does not
correspond to the current kernel value of 52 since kernel commit
6f0e8bf16730a36ff6773802d8c8df56d10e60cd (xen: support 52 bit
physical addresses in pv guests).
The result is visible in the above backtrace:
x86_64_pud_offset() is called with pgd_pte=0x3003a00e067 and that
value is wrongly masked by "pud_paddr = pgd_pte &
PHYSICAL_PAGE_MASK" and passed to xen_m2p() as 0x3a00e000 instead
of 0x3003a00e000, causing the m2p translation to fail.
Setting __PHYSICAL_MASK_SHIFT_XEN to 52 fixes the problem with
this dump.
But I am not confident it's a safe change. My understanding is
that it should be safe, as all the unused bits of the physical
address inside the PTEs should be 0 and thus having the mask wider
than necessary should not hurt. But I am suspicious if my
reasoning is correct. Why does crash go into such trouble
differentiating between different kernels and sets
machdep->machspec->physical_mask_shift dynamically to one of
__PHYSICAL_MASK_SHIFT_XEN (40), __PHYSICAL_MASK_SHIFT_2_6 (46),
and __PHYSICAL_MASK_SHIFT_5LEVEL (52)? Would something break if
it were always set to 52? The commit adding the logic is
307e7f35.
Thanks,
--
Jiri Bohac <jbohac(a)suse.cz>
SUSE Labs, Prague, Czechia