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Date: Wed, 18 Jun 2008 12:33:41 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Mike Travis <travis@sgi.com>
CC: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       Christoph Lameter <clameter@sgi.com>,
       David Miller <davem@davemloft.net>, Eric Dumazet <dada1@cosmosbay.com>,
       linux-kernel@vger.kernel.org, the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area
References: <20080604003018.538497000@polaris-admin.engr.sgi.com> <20080604003019.509483000@polaris-admin.engr.sgi.com> <20080605102222.GA21319@elte.hu> <484EF29C.7080100@sgi.com> <485947A8.8060801@goop.org> <4859511E.5050605@sgi.com>
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Mike Travis wrote:
> Jeremy Fitzhardinge wrote:
>   
>> 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
>>     
>
> Hi Jeremy,
>
> I'm not finding that code in the tip/latest or linux-next branches... ?
>   

You mean your percpu/pda code?  No, I'm carrying it locally because I 
need it as a base for my Xen work.  Xen bypasses these early boot 
stages, so I haven't seen any problems so far.
But I'd also like to make sure that my Xen changes don't break native 
boots, too...

> I can send you my latest version of the patch which is better than
> the previous but still is having problems with the config file that
> Ingo sent out.  (It also has a weird quirk that it will hang and
> reboot after about 30 seconds with or without my patch.)
>   

Yes, keep me uptodate with the percpu work.

    J
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