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

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Date:	Sun, 1 Nov 2009 23:49:06 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jan Kiszka <jan.kiszka@....de>
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

On Sun, Nov 01, 2009 at 11:09:03PM +0100, Jan Kiszka wrote:
> Frederic Weisbecker wrote:
> [...]
> > @@ -94,6 +87,12 @@ static inline void hw_breakpoint_disable(void)
> >  	set_debugreg(0UL, 3);
> >  }
> >
> > +#ifdef CONFIG_KVM
> > +extern void hw_breakpoint_restore(void);
> > +#else
> > +static inline void hw_breakpoint_restore(void)	{ }
> > +#endif
> > +
> 
> Hmm, if this shall become a KVM-only API, why the empty wrapper?



Right, this is an odd reflex.


 
> [...]
> > -void arch_flush_thread_hw_breakpoint(struct task_struct *tsk)
> > +#ifdef CONFIG_KVM
> > +void hw_breakpoint_restore(void)
> >  {
> > -	int i;
> > -	struct thread_struct *thread = &(tsk->thread);
> > -
> > -	thread->debugreg7 = 0;
> > -	for (i = 0; i < HBP_NUM; i++)
> > -		thread->debugreg[i] = 0;
> > +	set_debugreg(__get_cpu_var(cpu_debugreg[0]), 0);
> > +	set_debugreg(__get_cpu_var(cpu_debugreg[1]), 1);
> > +	set_debugreg(__get_cpu_var(cpu_debugreg[2]), 2);
> > +	set_debugreg(__get_cpu_var(cpu_debugreg[3]), 3);
> > +	set_debugreg(current->thread.debugreg6, 6);
> > +	set_debugreg(__get_cpu_var(dr7), 7);
> >  }
> > +#endif
> 
> EXPORT_SYMBOL_GPL(hw_breakpoint_restore);
> 
> as KVM can be built as module.



Indeed.


 
> [...]
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index fc2974a..32d7bca 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 +3644,8 @@ 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);
> > -	}
> > +	if (unlikely(vcpu->arch.switch_db_regs || test_thread_flag(TIF_DEBUG)))
> > +		hw_breakpoint_restore();
> 
> TIF_DEBUG is only set on active ptrace hw-breakpoints, thus we miss
> other types here, right? (Note: arch.switch_db_regs is guest-related,
> thus does not help in this regard.)
> 
> Jan
> 


True :)
Thanks a lot for this review, I'm going to fix that quickly.

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