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    SubjectRe: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events
    Frederic Weisbecker wrote:
    > 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.
    >

    And what about (__get_cpu_var(dr7) & DR_GLOBAL_ENABLE_MASK) only? Would
    you be able to live with unsync'ed hardware and software states?

    Jan

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