Re: [Xen-devel] Single step in HVM domU on Intel machine may see wrong DB6

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.02.14 at 02:31, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> wrote:
> Jan Beulich wrote on 2014-02-27:
>>>>> On 26.02.14 at 06:15, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> 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.

And presumably we would be better off if we dropped the
interception of TRAP_debug once we restored the debug
registers.

Jan


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