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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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