linux-kernel - Re: [RFC Patch 0/9] Hardware Breakpoint interfaces

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Date:	Tue, 07 Oct 2008 18:52:28 +0200
From:	Avi Kivity <avi@...hat.com>
To:	prasad@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org,
	Alan Stern <stern@...land.harvard.edu>,
	Roland McGrath <roland@...hat.com>, akpm@...ux-foundation.org,
	mingo@...e.hu, jason.wessel@...driver.com,
	richardj_moore@...ibm.com
Subject: Re: [RFC Patch 0/9] Hardware Breakpoint interfaces

K.Prasad wrote:
>>
>>> Presently I find plenty of set_debugreg() calls from kvm/ which will
>>> modify the registers directly and break the breakpoint register
>>> management brought-in through the patch.
>>>   
>>>       
>> If kvm restores the registers, should there be any problem?
>>
>>     
> No, it should be fine although I don't understand how the exception
> handler is invoked in KVM without the use of notifier or a hook in
> die_debug (or have they replaced code at a layer much below that?).
>
>   

kvm debug traps don't work by raising a debug exception; instead, the 
guest #VMEXITs into the host.  The kernel debug handler will not be invoked.

> Apart from the doubt I've stated above, if they operate by replacing
> the breakpoint register contents before a context switch from KVM to
> other processes, they might in fact help maximise its utilisation.
>   

Yes.  As I mentioned, right now we avoid swapping the debug registers if 
we aren't going to context switch or exit to userspace, but if you give 
us a predicate that tells us whether you are interested in the debug 
registers, we can swap them eagerly rather than lazily.

-- 
error compiling committee.c: too many arguments to function

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