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

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Date:	Mon, 2 Nov 2009 00:37:01 +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:
> > @@ -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
> 


About this. vcpu->arch.switch_db_regs is guest related but it looks
like the only thing I need to check.

I'm not sure when it is activated. Is it always done once the guest
changes its debug registers? I suspect there is a corner case.

Because since I can't anymore assume TIF_DEBUG covers every
breakpoints uses, it means I'll need to maintain a refcount of
breakpoints in use.
Well, I have one already, but it is splitted into several refcounts
(per task events, per cpu, non-pinned, etc...). And since
vcpu_enter_guest() is a fast path, I'll need to maintain another global
per cpu one, without lock or further operations to know if we need
to save the debug registers, just a simple check.

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