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