Re: Single step in HVM domU on Intel machine may see wrong DB6

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

On 05.03.2014 03:22, Zhang, Yang Z wrote:
> Jan Beulich wrote on 2014-02-27:
>>>>> On 27.02.14 at 02:31, "Zhang, Yang Z" <yang.z.zhang@intel.com> wrote:
>>> Jan Beulich wrote on 2014-02-27:
>>>>>>> On 26.02.14 at 06:15, "Zhang, Yang Z" <yang.z.zhang@intel.com>
>> wrote:
>>>>> @@ -2690,9 +2688,13 @@ void vmx_vmexit_handler(struct
>> cpu_user_regs
>>>> *regs)
>>>>>               __vmread(EXIT_QUALIFICATION, &exit_qualification);
>>>>>               HVMTRACE_1D(TRAP_DEBUG, exit_qualification);
>>>>>               write_debugreg(6, exit_qualification | 0xffff0ff0);
>>>>> -            if ( !v->domain->debugger_attached ||
>>>>> cpu_has_monitor_trap_flag ) -                goto exit_and_crash; -
>>>>>         domain_pause_for_debugger(); +            if (
>>>>> v->domain->debugger_attached ) +
>>>>> domain_pause_for_debugger(); +            else +            { +
>>>>>         __restore_debug_registers(v); +
>>>>> hvm_inject_hw_exception(TRAP_debug,
>> HVM_DELIVER_NO_ERROR_CODE); +
>>>>>       }
>>>>
>>>> I suppose you need to set DR6.BS after restoring the reigsters?
>>>
>>> Right but is not enough. If flag_dr_dirty is set, we need to restore
>>> register from hardware. Conversely, restore is from debugreg and set
>>> DR6 to exit_qualification.
>>
>> After some more thought, I in fact doubt that restoring the debug
>> registers is in line with the current model: We should simply set
>> DR6.BS in the in-memory copy when the debug registers aren't live yet
>> (and it doesn't hurt to always do that). And since DR6 bits generally
>> are sticky, I think exit_qualification actually needs to be or-ed into the in-memory copy.
>
> Will guest be confused to see the DR6.BS always set?

You can't set DR6.BS unconditionally! This bit should be set only in case
of a debug trap caused by single stepping, of course!

At least our BS2000 domU will crash in case of an unmotivated DR6.BS in debug
trap handling.


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
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