Messages in this thread | | | Date | Tue, 3 Nov 2009 21:15:06 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events |
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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.
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