Re: [Xen-devel] x86_64: Fix double fault stack setup

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/05/12 16:14, Jan Beulich wrote:
>>>> On 24.05.12 at 16:59, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>> x86_64: Fix double fault stack setup.
>>
>> Dont forget to push error_code and entry_vector onto the stack for a double
>> fault.  If it is missed, the register information printed looks like
>>
>> (XEN) CPU:    0
>> (XEN) RIP:    0246:[<000000000000e008>] ???
>> (XEN) RFLAGS: ffff82c480287eb8
>> (XEN) rax: 0000000000000282   rbx: ffff82c480242dd0   rcx: 0000000000000282
>> (XEN) rdx: 0000000000000000   rsi: 0000000000000282   rdi: 0000000000000031
>> (XEN) rbp: 0000000000000031   rsp: 0000000000000000   r8:  ffff83007ee52488
>> (XEN) r9:  ffff83007ee61088   r10: 0000000000000007   r11: ffff82c480116460
>> (XEN) r12: 0000000000000000   r13: ffff82c4802c37e0   r14: 00026501a9ced0b8
>> (XEN) r15: ffff82c4802c37c0    cs: 0000000000000246    ss: 0000000000000000
>>
>> which incorrectly displays cs, rip, rflags and rsp; the useful pieces of
>> information when trying to identify the cause of a double fault.
> Is this from an actual double fault, or from one of your INT 08
> attempts to simulate one? An actual exception pushes an error
> code, so I'm afraid the change below is wrong.
>
> Jan

Ah yes - how silly of me.  I misread the manual when checking that fact,
but this was an INT 08 experiment.  I really should have checked with a
ud2 as well.

That is a bit awkward.

Do we actually care about this error from an INT 08?  I suppose we could
check under rip for 0xcd 0x08, but then the same argument would apply to
all other exceptions which may push an error onto the stack.

Do we care however that entry_vector is not being set correctly?  I cant
see anything on the current codepath which uses it, but it doesn't
preclude someone adding code in the future.

~Andrew

>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>
>> diff -r 69c3ae25bb1d xen/arch/x86/x86_64/entry.S
>> --- a/xen/arch/x86/x86_64/entry.S
>> +++ b/xen/arch/x86/x86_64/entry.S
>> @@ -595,6 +595,8 @@ ENTRY(spurious_interrupt_bug)
>>          jmp   handle_exception
>>  
>>  ENTRY(double_fault)
>> +        pushq $0
>> +        movl $TRAP_double_fault,4(%rsp)
>>          SAVE_ALL
>>          movq  %rsp,%rdi
>>          call  do_double_fault
>>
>> -- 
>> Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
>> T: +44 (0)1223 225 900, http://www.citrix.com 
>
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx
> http://lists.xen.org/xen-devel

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com


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