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

From: Jan Kiszka <jan.kiszka@web.de>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>,
	Prasad <prasad@linux.vnet.ibm.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Peter Zijlstra <peterz@infradead.org>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Jiri Slaby <jirislaby@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>,
	Avi Kivity <avi@redhat.com>, Paul Mackerras <paulus@samba.org>,
	Mike Galbraith <efault@gmx.de>,
	Masami Hiramatsu <mhiramat@redhat.com>,
	Paul Mundt <lethal@linux-sh.org>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events
Date: Tue, 03 Nov 2009 21:39:16 +0100	[thread overview]
Message-ID: <4AF094F4.9060108@web.de> (raw)
In-Reply-To: <20091103202923.GB4962@nowhere>

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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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