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

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Message-ID: <09f2f813-3380-4567-0392-92c553ff3ca6@redhat.com>
Date:   Wed, 26 Oct 2016 09:35:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org
Cc:     Wanpeng Li <wanpeng.li@...mail.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] x86/msr: Add write msr notrace to avoid the debug
 codes splash



On 26/10/2016 06:21, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> 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@...radead.org>
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Mike Galbraith <efault@....de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  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?!?

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
>  	 */
>