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

Frederic Weisbecker
2009-11-01T23:36:58Z

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.