Ingo Molnar b20aeccd6a xen: fix early bootup crash on native hardware ... -tip tree auto-testing found the following early bootup hang: --------------> get_memcfg_from_srat: assigning address to rsdp RSD PTR v0 [Nvidia] BUG: Int 14: CR2 ffd00040 EDI 8092fbfe ESI ffd00040 EBP 80b0aee8 ESP 80b0aed0 EBX 000f76f0 EDX 0000000e ECX 00000003 EAX ffd00040 err 00000000 EIP 802c055a CS 00000060 flg 00010006 Stack: ffd00040 80bc78d0 80b0af6c 80b1dbfe 8093d8ba 00000008 80b42810 80b4ddb4 80b42842 00000000 80b0af1c 801079c8 808e724e 00000000 80b42871 802c0531 00000100 00000000 0003fff0 80b0af40 80129999 00040100 00040100 00000000 Pid: 0, comm: swapper Not tainted 2.6.26-rc4-sched-devel.git #570 [<802c055a>] ? strncmp+0x11/0x25 [<80b1dbfe>] ? get_memcfg_from_srat+0xb4/0x568 [<801079c8>] ? mcount_call+0x5/0x9 [<802c0531>] ? strcmp+0xa/0x22 [<80129999>] ? printk+0x38/0x3a [<80129999>] ? printk+0x38/0x3a [<8011b122>] ? memory_present+0x66/0x6f [<80b216b4>] ? setup_memory+0x13/0x40c [<80b16b47>] ? propagate_e820_map+0x80/0x97 [<80b1622a>] ? setup_arch+0x248/0x477 [<80129999>] ? printk+0x38/0x3a [<80b11759>] ? start_kernel+0x6e/0x2eb [<80b110fc>] ? i386_start_kernel+0xeb/0xf2 ======================= <------ with this config: http://redhat.com/~mingo/misc/config-Wed_May_28_01_33_33_CEST_2008.bad The thing is, the crash makes little sense at first sight. We crash on a benign-looking printk. The code around it got changed in -tip but checking those topic branches individually did not reproduce the bug. Bisection led to this commit: | d5edbc1f75 is first bad commit | commit d5edbc1f75 | Author: Jeremy Fitzhardinge <jeremy@goop.org> | Date: Mon May 26 23:31:22 2008 +0100 | | xen: add p2m mfn_list_list Which is somewhat surprising, as on native hardware Xen client side should have little to no side-effects. After some head scratching, it turns out the following happened: randconfig enabled the following Xen options: CONFIG_XEN=y CONFIG_XEN_MAX_DOMAIN_MEMORY=8 # CONFIG_XEN_BLKDEV_FRONTEND is not set # CONFIG_XEN_NETDEV_FRONTEND is not set CONFIG_HVC_XEN=y # CONFIG_XEN_BALLOON is not set which activated this piece of code in arch/x86/xen/mmu.c: > @@ -69,6 +69,13 @@ > __attribute__((section(".data.page_aligned"))) = > { [ 0 ... TOP_ENTRIES - 1] = &p2m_missing[0] }; > > +/* Arrays of p2m arrays expressed in mfns used for save/restore */ > +static unsigned long p2m_top_mfn[TOP_ENTRIES] > + __attribute__((section(".bss.page_aligned"))); > + > +static unsigned long p2m_top_mfn_list[TOP_ENTRIES / P2M_ENTRIES_PER_PAGE] > + __attribute__((section(".bss.page_aligned"))); The problem is, you must only put variables into .bss.page_aligned that have a _size_ that is _exactly_ page aligned. In this case the size of p2m_top_mfn_list is not page aligned: 80b8d000 b p2m_top_mfn 80b8f000 b p2m_top_mfn_list 80b8f008 b softirq_stack 80b97008 b hardirq_stack 80b9f008 b bm_pte So all subsequent variables get unaligned which, depending on luck, breaks the kernel in various funny ways. In this case what killed the kernel first was the misaligned bootmap pte page, resulting in that creative crash above. Anyway, this was a fun bug to track down :-) I think the moral is that .bss.page_aligned is a dangerous construct in its current form, and the symptoms of breakage are very non-trivial, so i think we need build-time checks to make sure all symbols in .bss.page_aligned are truly page aligned. The Xen fix below gets the kernel booting again. Signed-off-by: Ingo Molnar <mingo@elte.hu>		2008-05-28 14:32:06 +02:00
..
boot	x86: relocs ELF handling - use SELFMAG instead of numeric constant	2008-05-04 20:04:45 +02:00
configs
crypto
ia32	signals: x86 TS_RESTORE_SIGMASK	2008-04-30 08:29:37 -07:00
kernel	x86/paravirt: add pte_flags to just get pte flags	2008-05-27 10:11:36 +02:00
kvm	KVM: LAPIC: ignore pending timers if LVTT is disabled	2008-05-18 14:39:39 +03:00
lguest
lib	x86: fix csum_partial() export	2008-05-13 19:38:47 +02:00
mach-default
mach-es7000
mach-generic
mach-rdc321x
mach-visws
mach-voyager	x86, voyager: fix ioremap_nocache()	2008-04-30 23:15:34 +02:00
math-emu
mm	x86: fix crash on cpu hotplug on pat-incapable machines	2008-05-17 22:57:20 +02:00
oprofile
pci	PCI: Correct last two HP entries in the bfsort whitelist	2008-05-19 12:21:36 -07:00
power
vdso	x86: vdso ELF handling - use SELFMAG instead of numeric constant	2008-05-04 20:04:45 +02:00
video	x86: video/fbdev.c: add MODULE_LICENSE	2008-05-04 20:04:46 +02:00
xen	xen: fix early bootup crash on native hardware	2008-05-28 14:32:06 +02:00
Kconfig	x86: rdc: leds build/config fix	2008-05-10 19:31:45 +02:00
Kconfig.cpu	x86: CONFIG_X86_ELAN fix	2008-04-30 23:15:35 +02:00
Kconfig.debug	x86: Mark OPTIMIZE_INLINING broken	2008-04-30 20:07:22 -07:00
Makefile
Makefile_32.cpu