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

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Message-ID: <48EB7406.1010509@redhat.com>
Date:	Tue, 07 Oct 2008 16:36:54 +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:
> On Tue, Oct 07, 2008 at 02:29:05PM +0200, Avi Kivity wrote:
>   
>> K.Prasad wrote:
>>     
>>> - Enable KGDB and KVM to use the register_kernel_hw_breakpoint()
>>>   interface for their HW Breakpoint usage, in the absence of which
>>>   they will be broken during simultaneous use.
>>>   
>>>       
>> KVM conceptually isn't a kernel use of the debug registers.  KVM  
>> modifies the debug registers while the guest is running, and restores  
>> them after the guest returns.
>>
>> Right now, as an optimization, KVM defers restoring the debug registers  
>> until after the next context switch out of the kvm task, or until the  
>> next exit to userspace (whichever comes earlier); we should change this  
>> to avoid the deferral if kernel breakpoints are in effect.  This will  
>> allow simultaneous use of KVM breakpoints and kernel breakpoints.
>>
>>     
> The patch posted provides interface for using HW breakpoints on both
> user- and kernel-space breakpoints. KVM's user-space address breakpoint
> requirements, after code-modification, should be made to use
> register_user_hw_breakpoint() interface (presently un-exported but will
> be changed subsequently) to help maintain a system-wide consistent view on the
> availability of HW Breakpoint registers.
>
>   

Correcting myself, actually kvm breakpoints are in a third namespace.  
You could have kernel breakpoints, user breakpoints, and guest 
breakpoints coexisting.

> 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?

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

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