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

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Message-ID: <20081007143217.GA3774@in.ibm.com>
Date:	Tue, 7 Oct 2008 20:02:17 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.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

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.

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.

Thanks,
K.Prasad

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