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 21:22:04 +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 08:58:52PM +0100, Jan Kiszka wrote:
>> Frederic Weisbecker wrote:
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index fc2974a..6560129 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -42,6 +42,7 @@
>>>  #define CREATE_TRACE_POINTS
>>>  #include "trace.h"
>>>  
>>> +#include <asm/debugreg.h>
>>>  #include <asm/uaccess.h>
>>>  #include <asm/msr.h>
>>>  #include <asm/desc.h>
>>> @@ -3643,14 +3644,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>>>  	trace_kvm_entry(vcpu->vcpu_id);
>>>  	kvm_x86_ops->run(vcpu, kvm_run);
>>>  
>>> -	if (unlikely(vcpu->arch.switch_db_regs || test_thread_flag(TIF_DEBUG))) {
>>> -		set_debugreg(current->thread.debugreg[0], 0);
>>> -		set_debugreg(current->thread.debugreg[1], 1);
>>> -		set_debugreg(current->thread.debugreg[2], 2);
>>> -		set_debugreg(current->thread.debugreg[3], 3);
>>> -		set_debugreg(current->thread.debugreg6, 6);
>>> -		set_debugreg(current->thread.debugreg7, 7);
>>> -	}
>>> +	/*
>>> +	 * CHECKME: is vcpu->arch.switch_db_regs sufficient to check
>>> +	 * if the guest is using breakpoints? If so we may want to do
>>> +	 * this check before.
>>> +	 */
>>> +	hw_breakpoint_restore();
>> Obviously, this variant will make KVM users very unhappy. But trying to
>> reduce this performance regression via vcpu->arch.switch_db_regs will
>> make hw-breakpoint users unhappy: KVM leaves at least dr7 clobbered
>> behind, even if the guest does not use breakpoints.
> 
> 
> Yeah, that's why I've made unconditionally. At least it works in every
> cases, but this is temporary.
> 
>  
>> We really need a replacement for TIF_DEBUG (but we only need this [1]).
>>
>> Jan
>>
>> [1]http://thread.gmane.org/gmane.comp.emulators.kvm.devel/39784/focus=39827
>>
> 
> 
> 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


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