linux-kernel - Re: [PATCH] KVM: lapic: sync highest ISR to hardware apic on EOI

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Message-ID: <53835636.5050801@redhat.com>
Date:	Mon, 26 May 2014 16:56:54 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	linux-kernel@...r.kernel.org, yang.z.zhang@...el.com,
	mtosatti@...hat.com, stable@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: lapic: sync highest ISR to hardware apic on EOI

Il 26/05/2014 16:28, Michael S. Tsirkin ha scritto:
>>  static inline void apic_clear_isr(int vec, struct kvm_lapic *apic)
>>  {
>> -	if (__apic_test_and_clear_vector(vec, apic->regs + APIC_ISR))
>> +	struct kvm_vcpu *vcpu;
>> +	if (!__apic_test_and_clear_vector(vec, apic->regs + APIC_ISR))
>> +		return;
>> +
>> +	vcpu = apic->vcpu;
>> +
>> +	/*
>> +	 * We do get here for APIC virtualization enabled if the guest
>> +	 * uses the Hyper-V APIC enlightenment.  In this case we may need
>> +	 * to trigger a new interrupt delivery by writing the SVI field;
>> +	 * on the other hand isr_count and highest_isr_cache are unused
>> +	 * and must be left alone.
>> +	 */
>> +	if (unlikely(kvm_apic_vid_enabled(vcpu->kvm)))
>> +		kvm_x86_ops->hwapic_isr_update(vcpu->kvm,
>> +					       apic_find_highest_isr(apic));
>
> I note that an indirect call on data path is mostly unnecessary here:
> 	static int vmx_vm_has_apicv(struct kvm *kvm)
> 	{
> 		return enable_apicv && irqchip_in_kernel(kvm);
> 	}
> is all it does, and irqchip_in_kernel also has an rmb within it, which
> is somewhat expensive on x86: and there's no way to reach this code
> with irqchip disabled, correct?

smp_rmb is just a compiler barrier, it's not expensive.  The indirect 
call is probably more expensive.

That said, other places in the paths (kvm_cpu_has_injectable_intr, 
kvm_cpu_get_interrupt) already call kvm_apic_vid_enabled, so this patch 
doesn't introduce something new.

> How about adding a bool flag in kvm_vcpu_arch, and testing that?

Yes, that's possible.  It's also possible to test 
kvm_x86_ops->hwapic_isr_update != NULL and zero the field in vmx.c's 
hardware_setup.  Can you make a patch?

Paolo
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