linux-kernel - Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

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Date:	Tue, 03 Nov 2009 21:39:16 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer
 on top of perf events

Frederic Weisbecker wrote:
> On Tue, Nov 03, 2009 at 09:22:04PM +0100, Jan Kiszka wrote:
>>> Thinking about it, this check should cover every cases:
>>>
>>> if (vcpu->arch.switch_db_regs || __get_cpu_var(dr7) & DR_GLOBAL_ENABLE_MASK)
>>>
>>> If we have __get_cpu_var(dr7) & DR_GLOBAL_ENABLE_MASK, it means there is an
>>> active breakpoint and then we should restore the current state.
>>>
>> And what about (__get_cpu_var(dr7) & DR_GLOBAL_ENABLE_MASK) only? Would
>> you be able to live with unsync'ed hardware and software states?
>>
>> Jan
>>
> 
> 
> But if the guest has breakpoints activated, the host will inherit
> them, which is really not something we want, assuming vcpu->arch.switch_db_regs
> already protects us about that.
> 

Nope, vmx&svm will clear dr7 on vmexit for us. Really, switch_db_regs
should not be needed if we can leave the debug registers clobbered but
disabled behind.

Jan


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