Re: [PATCH 3/3] KVM: x86: make vapics_in_nmi_mode atomic

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/06/2015 22:19, Radim Krčmář wrote:
> Writes were a bit racy, but hard to turn into a bug at the same time.
> (Particularly because modern Linux doesn't use this feature anymore.)

I suspect patch 2 makes this race much easier to trigger, so it deserves
Cc: stable@ as well.

Paolo

> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  arch/x86/kvm/i8254.c            | 2 +-
>  arch/x86/kvm/lapic.c            | 4 ++--
>  3 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c7fa57b529d2..2a7f5d782c33 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -607,7 +607,7 @@ struct kvm_arch {
>  	struct kvm_pic *vpic;
>  	struct kvm_ioapic *vioapic;
>  	struct kvm_pit *vpit;
> -	int vapics_in_nmi_mode;
> +	atomic_t vapics_in_nmi_mode;
>  	struct mutex apic_map_lock;
>  	struct kvm_apic_map *apic_map;
>  
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index 4dce6f8b6129..f90952f64e79 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -305,7 +305,7 @@ static void pit_do_work(struct kthread_work *work)
>  		 * LVT0 to NMI delivery. Other PIC interrupts are just sent to
>  		 * VCPU0, and only if its LVT0 is in EXTINT mode.
>  		 */
> -		if (kvm->arch.vapics_in_nmi_mode > 0)
> +		if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0)
>  			kvm_for_each_vcpu(i, vcpu, kvm)
>  				kvm_apic_nmi_wd_deliver(vcpu);
>  	}
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 8dc32b5a4e0d..954e98a8c2e3 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1264,9 +1264,9 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
>  		if (lvt0_in_nmi_mode) {
>  			apic_debug("Receive NMI setting on APIC_LVT0 "
>  				   "for cpu %d\n", apic->vcpu->vcpu_id);
> -			apic->vcpu->kvm->arch.vapics_in_nmi_mode++;
> +			atomic_inc(&apic->vcpu->kvm->arch.vapics_in_nmi_mode);
>  		} else
> -			apic->vcpu->kvm->arch.vapics_in_nmi_mode--;
> +			atomic_dec(&apic->vcpu->kvm->arch.vapics_in_nmi_mode);
>  	}
>  }
>  
>