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 14:04:40 +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 Mon, Nov 02, 2009 at 08:45:56AM +0100, Jan Kiszka wrote:
> Frederic Weisbecker wrote:
> > 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.
> 
> It's set when we had to write to some debugreg[0..4], either for use by
> the guest itself or for debugging it from the host. It used to be the
> only condition for switching on exit as we saved the registers on entry
> (under the same condition). This was reworked recently to avoid the
> entry saving.
> 
> > 
> > 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.
> > 
> 
> I'm not 100% sure right now if we still need "switch_db_reg" in case we
> have a reliable indicator that the host requires properly set registers.
> ATM I would dare to say, we don't, but I need to think about this again.
> 
> Jan
> 


Ok. I'm going to just check if the host has active breakpoints pending and
if so, I'll restore them while exiting the guest.

Thanks.

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