Re: [PATCH v4] x86/msr: Add write msr notrace to avoid the debug codes splash

[Wanpeng Li]

2016-10-26 15:35 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 26/10/2016 06:21, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> As Peterz pointed out:
>>
>> | The thing is, many many smp_reschedule_interrupt() invocations don't
>> | actually execute anything much at all and are only send to tickle the
>> | return to user path (which does the actual preemption).
>>
>> This patch add write msr notrace to avoid the debug codes splash.
>>
>> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Mike Galbraith <efault@xxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/apic.h |  3 ++-
>>  arch/x86/include/asm/msr.h  | 15 +++++++++++++++
>>  arch/x86/kernel/apic/apic.c |  1 +
>>  arch/x86/kernel/kvm.c       |  6 +++---
>>  arch/x86/kernel/smp.c       |  2 --
>>  5 files changed, 21 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
>> index f5aaf6c..a5a0bcf 100644
>> --- a/arch/x86/include/asm/apic.h
>> +++ b/arch/x86/include/asm/apic.h
>> @@ -196,7 +196,7 @@ static inline void native_apic_msr_write(u32 reg, u32 v)
>>
>>  static inline void native_apic_msr_eoi_write(u32 reg, u32 v)
>>  {
>> -     wrmsr(APIC_BASE_MSR + (APIC_EOI >> 4), APIC_EOI_ACK, 0);
>> +     wrmsr_notrace(APIC_BASE_MSR + (APIC_EOI >> 4), APIC_EOI_ACK, 0);
>>  }
>>
>>  static inline u32 native_apic_msr_read(u32 reg)
>> @@ -332,6 +332,7 @@ struct apic {
>>        * on write for EOI.
>>        */
>>       void (*eoi_write)(u32 reg, u32 v);
>> +     void (*native_eoi_write)(u32 reg, u32 v);
>>       u64 (*icr_read)(void);
>>       void (*icr_write)(u32 low, u32 high);
>>       void (*wait_icr_idle)(void);
>> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
>> index b5fee97..afbb221 100644
>> --- a/arch/x86/include/asm/msr.h
>> +++ b/arch/x86/include/asm/msr.h
>> @@ -127,6 +127,21 @@ notrace static inline void native_write_msr(unsigned int msr,
>>  }
>>
>>  /* Can be uninlined because referenced by paravirt */
>> +notrace static inline void native_write_msr_notrace(unsigned int msr,
>> +                                         unsigned low, unsigned high)
>> +{
>> +     asm volatile("1: wrmsr\n"
>> +                  "2:\n"
>> +                  _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
>> +                  : : "c" (msr), "a"(low), "d" (high) : "memory");
>> +}
>> +
>> +static inline void wrmsr_notrace(unsigned msr, unsigned low, unsigned high)
>> +{
>> +     native_write_msr_notrace(msr, low, high);
>> +}
>> +
>> +/* Can be uninlined because referenced by paravirt */
>>  notrace static inline int native_write_msr_safe(unsigned int msr,
>>                                       unsigned low, unsigned high)
>>  {
>> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
>> index 88c657b..2686894 100644
>> --- a/arch/x86/kernel/apic/apic.c
>> +++ b/arch/x86/kernel/apic/apic.c
>> @@ -2263,6 +2263,7 @@ void __init apic_set_eoi_write(void (*eoi_write)(u32 reg, u32 v))
>>       for (drv = __apicdrivers; drv < __apicdrivers_end; drv++) {
>>               /* Should happen once for each apic */
>>               WARN_ON((*drv)->eoi_write == eoi_write);
>> +             (*drv)->native_eoi_write = (*drv)->eoi_write;
>>               (*drv)->eoi_write = eoi_write;
>>       }
>>  }
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index edbbfc8..a4627ed 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -308,7 +308,7 @@ static void kvm_register_steal_time(void)
>>
>>  static DEFINE_PER_CPU(unsigned long, kvm_apic_eoi) = KVM_PV_EOI_DISABLED;
>>
>> -static void kvm_guest_apic_eoi_write(u32 reg, u32 val)
>> +static void kvm_guest_apic_eoi_write_notrace(u32 reg, u32 val)
>
> Huh, this is not notrace?!?

Sorry, maybe I misunderstand you.

ack_APIC_irq()
  -> apic_eoi()
    -> apic->eoi_write()
      -> kvm_guest_apic_eoi_write()
        -> apic->native_eoi_write()
          -> native_apic_msr_eoi_write()
            -> wrmsr_notrace()
              -> native_write_msr_notrace()
          -> native_apic_mem_write()

The call path is notrace. Where I miss?

Regards,
Wanpeng Li

>
> Paolo
>
>>  {
>>       /**
>>        * This relies on __test_and_clear_bit to modify the memory
>> @@ -319,7 +319,7 @@ static void kvm_guest_apic_eoi_write(u32 reg, u32 val)
>>        */
>>       if (__test_and_clear_bit(KVM_PV_EOI_BIT, this_cpu_ptr(&kvm_apic_eoi)))
>>               return;
>> -     apic_write(APIC_EOI, APIC_EOI_ACK);
>> +     apic->native_eoi_write(APIC_EOI, APIC_EOI_ACK);
>>  }
>>
>>  static void kvm_guest_cpu_init(void)
>> @@ -474,7 +474,7 @@ void __init kvm_guest_init(void)
>>       }
>>
>>       if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>> -             apic_set_eoi_write(kvm_guest_apic_eoi_write);
>> +             apic_set_eoi_write(kvm_guest_apic_eoi_write_notrace);
>>
>>       if (kvmclock_vsyscall)
>>               kvm_setup_vsyscall_timeinfo();
>> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
>> index c00cb64..68f8cc2 100644
>> --- a/arch/x86/kernel/smp.c
>> +++ b/arch/x86/kernel/smp.c
>> @@ -261,10 +261,8 @@ static inline void __smp_reschedule_interrupt(void)
>>
>>  __visible void smp_reschedule_interrupt(struct pt_regs *regs)
>>  {
>> -     irq_enter();
>>       ack_APIC_irq();
>>       __smp_reschedule_interrupt();
>> -     irq_exit();
>>       /*
>>        * KVM uses this interrupt to force a cpu out of guest mode
>>        */
>>