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SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
Mike Travis wrote:
> Ingo Molnar wrote:
>
>> * Mike Travis <travis@sgi.com> wrote:
>>
>>
>>> * Declare the pda as a per cpu variable.
>>>
>>> * Make the x86_64 per cpu area start at zero.
>>>
>>> * Since the pda is now the first element of the per_cpu area, cpu_pda()
>>> is no longer needed and per_cpu() can be used instead. This also makes
>>> the _cpu_pda[] table obsolete.
>>>
>>> * Since %gs is pointing to the pda, it will then also point to the per cpu
>>> variables and can be accessed thusly:
>>>
>>> %gs:[&per_cpu_xxxx - __per_cpu_start]
>>>
>>> Based on linux-2.6.tip
>>>
>> -tip testing found an instantaneous reboot crash on 64-bit x86, with
>> this config:
>>
>> http://redhat.com/~mingo/misc/config-Thu_Jun__5_11_43_51_CEST_2008.bad
>>
>
> I'm still stuck on this one. One new development is that the current -tip
> branch without the patches boots to the kernel prompt then hangs after a few
> moments and then reboots. It seems you can tickle it using ^C to abort a
> process.

Hi Mike,

I added some instrumentation to Xen to print the cpu state on
triple-fault, which highlights an obvious-looking problem.

(XEN) hvm.c:767:d1 Triple fault on VCPU0 - invoking HVM system reset.
(XEN) ----[ Xen-3.3-unstable x86_64 debug=y Not tainted ]----
(XEN) CPU: 1
(XEN) RIP: 0010:[<ffffffff80200160>]
(XEN) RFLAGS: 0000000000010002 CONTEXT: hvm
(XEN) rax: 0000000000000018 rbx: 0000000000000000 rcx: 00000000c0000080
(XEN) rdx: 0000000000000000 rsi: 0000000000092f40 rdi: 0000000020100800
(XEN) rbp: 0000000000000000 rsp: ffffffff807dfff8 r8: 0000000000208000
(XEN) r9: 0000000000000000 r10: 0000000000000000 r11: 00000000000000de
(XEN) r12: 0000000000000000 r13: 0000000000000000 r14: 0000000000000000
(XEN) r15: 0000000000000000 cr0: 0000000080050033 cr4: 00000000000000a0
(XEN) cr3: 0000000000201000 cr2: 0000000000000000
(XEN) ds: 0000 es: 0000 fs: 0000 gs: 0000 ss: 0000 cs: 0010

The rip is:

(gdb) x/i 0xffffffff80200160
0xffffffff80200160 <secondary_startup_64+96>: movl %eax,%ds

which is:

lgdt early_gdt_descr(%rip)

/* set up data segments. actually 0 would do too */
movl $__KERNEL_DS,%eax
movl %eax,%ds
movl %eax,%ss
movl %eax,%es

And early_gdt_descr is:

.globl early_gdt_descr
early_gdt_descr:
.word GDT_ENTRIES*8-1
.quad per_cpu__gdt_page

and per_cpu__gdt_page is zero-based, and therefore not a directly
addressable symbol.

I tried this patch, but it didn't work. Perhaps I'm missing something.

diff -r bf5a46e13f78 arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S Tue Jun 17 22:10:51 2008 -0700
+++ b/arch/x86/kernel/head_64.S Wed Jun 18 10:34:24 2008 -0700
@@ -94,6 +94,8 @@

addq %rbp, level2_fixmap_pgt + (506*8)(%rip)

+ addq $__per_cpu_load, early_gdt_descr+2(%rip)
+
/* Add an Identity mapping if I am above 1G */
leaq _text(%rip), %rdi
andq $PMD_PAGE_MASK, %rdi

J


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