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SubjectRe: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area
Mike Travis wrote:
> Jeremy Fitzhardinge wrote:
>
>> Mike Travis wrote:
>>
>>> @@ -132,6 +133,12 @@ ident_complete:
>>> #ifdef CONFIG_SMP
>>> addq %rbp, trampoline_level4_pgt + 0(%rip)
>>> addq %rbp, trampoline_level4_pgt + (511*8)(%rip)
>>> +
>>> + /*
>>> + * Fix up per_cpu__gdt_page offset when basing percpu
>>> + * variables at zero. This is only needed for the boot cpu.
>>> + */
>>> + addq $__per_cpu_load, early_gdt_descr_base
>>>
>>>
>> This needs to be rip-relative. An absolute reference here will fail
>> because you're still running in physical addresses.
>>
>> J
>>
>
> Still bombs right at boot up... ;-(
>

Yep. I see the triple-fault at the "mov %eax,%ds", which means it's
having trouble with the gdt. Either 1) the lgdt pointed to a bad
address, or 2) there's something wrong with the descriptor there.

The dump is:

(XEN) hvm.c:767:d14 Triple fault on VCPU0 - invoking HVM system reset.
(XEN) ----[ Xen-3.3-unstable x86_64 debug=n Not tainted ]----
(XEN) CPU: 0
(XEN) RIP: 0010:[<ffffffff80200167>]
(XEN) RFLAGS: 0000000000010002 CONTEXT: hvm
(XEN) rax: 0000000000000018 rbx: 0000000000000000 rcx: ffffffff808d6000
(XEN) rdx: 0000000000000000 rsi: 0000000000092f40 rdi: 0000000020100800
(XEN) rbp: 0000000000000000 rsp: ffffffff80827ff8 r8: 0000000000208000
(XEN) r9: 0000000000000000 r10: 0000000000000000 r11: 00000000000000d8
(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


I loaded early_gdt_descr+2 into %rcx, which looks reasonable. Hm, but
loading the __KERNEL_DS descriptor into %rdx, which is all zero.

So it seems the problem is that the pre-initialized gdt_page is being
lost and replaced with zero. Linker script bug?

J

--- a/arch/x86/kernel/head_64.S Fri Jun 20 09:50:02 2008 -0700
+++ b/arch/x86/kernel/head_64.S Fri Jun 20 10:19:20 2008 -0700
@@ -213,6 +213,8 @@
* because in 32bit we couldn't load a 64bit linear address.
*/
lgdt early_gdt_descr(%rip)
+ movq early_gdt_descr+2(%rip), %rcx
+ movq __KERNEL_DS(%rcx), %rdx

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



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