Date:	Thu, 28 Jan 2010 14:27:54 -0600
From:	Jason Wessel <jason.wessel@...driver.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu, "K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH 1/4] x86,hw_breakpoint,kgdb: kgdb to usehw_breakpointAPI

Frederic Weisbecker wrote:
> On Thu, Jan 28, 2010 at 11:44:52AM -0600, Jason Wessel wrote:
>   
>> Frederic Weisbecker wrote:
>>     
>>> Good simplification, but that doesn't appear related to kgdb,
>>> this should be done in a separate patch, for the perf/core tree.
>>>
>>>   
>>>       
>> Specifically this is required so that kgdb can modify the state of dr7
>> by installing and removing breakpoints.  Without this change, on return
>> from the callback the dr7 was not correct.
>>
>> As far as I know, only kgdb was altering the dr registers during a call
>> back..
>>     
>
>
>
> Ok. Well not sure how/where it needs to modify dr7 directly.
>
>   

This is not needed any more with the patch I already followed up with. 


To provide an explanation though, dr7 got updated as a result of
installing some breakpoints while in kgdb while in the call back.   And
that value got squashed by what ever was saved on the stack as a local
in the perf breakpoint handler.

>  
>   
>> I admit I did not test running a kvm instance, so I don't know what kind
>> of conflict there would be here.  I went and further looked at the kvm
>> code, and they call the function for the same reason kgdb does.  They
>> want the original system values back on resuming normal kernel
>> execution.  KVM can modify dr7 or other regs directly on entry for its
>> guest execution.  Kgdb does the same sort of thing so as to prevent the
>> debugger from interrupting itself.
>>     
>
>
>
> You mean kgdb needs to disable dr7 while handling a breakpoint to
> avoid recursion? In this case this is something already done
> from the x86 breakpoint handler.
>
>
>   

True, that is done for the master core, but not the slave cores, which
we stop dead in their tracks with a nmi, so they have not had dr7 zeroed
out.

Obviously we restore it on resume.

>  
>   
>>> Would be nice to have bptype set to the generic flags
>>> we have already in linux/hw_breakpoint.h:
>>>
>>> enum {
>>> 	HW_BREAKPOINT_R = 1,
>>> 	HW_BREAKPOINT_W = 2,
>>> 	HW_BREAKPOINT_X = 4,
>>> };
>>>
>>>
>>>   
>>>       
>> These numbers have to get translated somewhere from the GDB version
>> which it handed off via the gdb serial protocol.  They could be
>> translated in the gdb stub, but for now they are in the arch specific
>> stub.  Or you can choose to use the same numbering scheme as gdb for the
>> breakpoint types and the values could be used directly.
>>     
>
>
>
> Ah ok. Well, translating gdb <-> generic values will make you
> move this code from x86 to core at least.
>
>   


I'll move this to the gdbstub in the next merge window, because it looks
like there might also be other archs that gain the
arch/*/kernel/hw_breakpoint.c.

I can also move the logic for the install / remove to be owned by either
hw_breakpoint.c or the debug core because that will become arch
independent as well as more archs pickup the hw_breakpoint.c methodology.


Thanks,
Jason.
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