答复: [PATCH][v2] KVM: Clear pv eoi pending bit only when it is set

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: Vitaly Kuznetsov <vkuznets@redhat.com>
> 发送时间: 2021年11月3日 23:12
> 收件人: Li,Rongqing <lirongqing@baidu.com>
> 抄送: Li,Rongqing <lirongqing@baidu.com>; pbonzini@redhat.com;
> seanjc@google.com; wanpengli@tencent.com; jmattson@google.com;
> joro@8bytes.org; tglx@linutronix.de; mingo@redhat.com; bp@alien8.de;
> x86@kernel.org; hpa@zytor.com; kvm@vger.kernel.org
> 主题: Re: [PATCH][v2] KVM: Clear pv eoi pending bit only when it is set
> 
> Li RongQing <lirongqing@baidu.com> writes:
> 
> > merge pv_eoi_get_pending and pv_eoi_clr_pending into a single function
> > pv_eoi_test_and_clear_pending, which returns and clear the value of
> > the pending bit.
> >
> > and clear pv eoi pending bit only when it is set, to avoid calling
> > pv_eoi_put_user(), this can speed about 300 nsec on AMD EPYC most of
> > the time
> >
> > and make pv_eoi_set_pending as inline as there is only one user
> 
> Compiler is likely smart enough to inline static functions with a single user
> anyway.
> 
> >
> > Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > ---
> > diff with v1:
> >  merge as pv_eoi_test_and_clear_pending  add inline for
> > pv_eoi_set_pending
> >
> >  arch/x86/kvm/lapic.c |   47 +++++++++++++++++++++++------------------------
> >  1 files changed, 23 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index
> > 76fb009..4da5db8 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -673,18 +673,7 @@ static inline bool pv_eoi_enabled(struct kvm_vcpu
> *vcpu)
> >  	return vcpu->arch.pv_eoi.msr_val & KVM_MSR_ENABLED;  }
> >
> > -static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu) -{
> > -	u8 val;
> > -	if (pv_eoi_get_user(vcpu, &val) < 0) {
> > -		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
> > -			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
> > -		return false;
> > -	}
> > -	return val & KVM_PV_EOI_ENABLED;
> > -}
> > -
> > -static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
> > +static inline void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
> >  {
> >  	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_ENABLED) < 0) {
> >  		printk(KERN_WARNING "Can't set EOI MSR value: 0x%llx\n", @@
> -694,14
> > +683,31 @@ static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
> >  	__set_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);  }
> >
> > -static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
> > +static inline bool pv_eoi_test_and_clr_pending(struct kvm_vcpu *vcpu)
> >  {
> > -	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0) {
> > +	u8 val;
> > +
> > +	if (pv_eoi_get_user(vcpu, &val) < 0) {
> > +		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
> > +			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
> 
> pr_warn() would probably be a better choice but looking at this makes me
> wonder: isn't it triggerable by the guest? I think it is when the value written to
> MSR_KVM_PV_EOI_EN is bogus and this is bad: we don't even ratelimit these
> messages! I think this printk() needs to be dropped.
> 

True, it needs to be removed.
And it is introduced by this below patch ; I think it should be a new patch to fix it. 


commit 0d88800d547211ce07be3551c812d404cf2be3a8
Author: Yi Wang <wang.yi59@zte.com.cn>
Date:   Sat Jul 6 01:08:48 2019 +0800

    kvm: x86: ioapic and apic debug macros cleanup


thanks

-Li


> > +		return false;
> > +	}
> > +
> > +	val &= KVM_PV_EOI_ENABLED;
> > +
> > +	/*
> > +	 * Clear pending bit in any case: it will be set again on vmentry.
> > +	 * While this might not be ideal from performance point of view,
> > +	 * this makes sure pv eoi is only enabled when we know it's safe.
> > +	 */
> > +	if (val && pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0) {
> >  		printk(KERN_WARNING "Can't clear EOI MSR value: 0x%llx\n",
> >  			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
> 
> ... and this one, probably, too.
> 
> > -		return;
> > +		return false;
> >  	}
> >  	__clear_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
> > +
> > +	return !!val;
> >  }
> >
> >  static int apic_has_interrupt_for_ppr(struct kvm_lapic *apic, u32
> > ppr) @@ -2673,7 +2679,6 @@ void __kvm_migrate_apic_timer(struct
> > kvm_vcpu *vcpu)  static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu
> *vcpu,
> >  					struct kvm_lapic *apic)
> >  {
> > -	bool pending;
> >  	int vector;
> >  	/*
> >  	 * PV EOI state is derived from KVM_APIC_PV_EOI_PENDING in host @@
> > -2687,14 +2692,8 @@ static void apic_sync_pv_eoi_from_guest(struct
> kvm_vcpu *vcpu,
> >  	 * 	-> host enabled PV EOI, guest executed EOI.
> >  	 */
> >  	BUG_ON(!pv_eoi_enabled(vcpu));
> > -	pending = pv_eoi_get_pending(vcpu);
> > -	/*
> > -	 * Clear pending bit in any case: it will be set again on vmentry.
> > -	 * While this might not be ideal from performance point of view,
> > -	 * this makes sure pv eoi is only enabled when we know it's safe.
> > -	 */
> > -	pv_eoi_clr_pending(vcpu);
> > -	if (pending)
> > +
> > +	if (pv_eoi_test_and_clr_pending(vcpu))
> >  		return;
> >  	vector = apic_set_eoi(apic);
> >  	trace_kvm_pv_eoi(apic, vector);
> 
> --
> Vitaly