linux-kernel - Re: [PATCH] KVM: SVM: allocate AVIC data structures based on kvm

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Date:   Tue, 25 Feb 2020 14:45:36 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     rmuncrief@...anavance.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: allocate AVIC data structures based on kvm_amd moduleparameter

Paolo Bonzini <pbonzini@...hat.com> writes:

> Even if APICv is disabled at startup, the backing page and ir_list need
> to be initialized in case they are needed later.  The only case in
> which this can be skipped is for userspace irqchip, and that must be
> done because avic_init_backing_page dereferences vcpu->arch.apic
> (which is NULL for userspace irqchip).
>
> Tested-by: rmuncrief@...anavance.com
> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=206579
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index ad3f5b178a03..bd02526300ab 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2194,8 +2194,9 @@ static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  static int avic_init_vcpu(struct vcpu_svm *svm)
>  {
>  	int ret;
> +	struct kvm_vcpu *vcpu = &svm->vcpu;
>  
> -	if (!kvm_vcpu_apicv_active(&svm->vcpu))
> +	if (!avic || !irqchip_in_kernel(vcpu->kvm))
>  		return 0;
>  
>  	ret = avic_init_backing_page(&svm->vcpu);

Out of pure curiosity,

when irqchip_in_kernel() is false, can we still get to .update_pi_irte()
(svm_update_pi_irte()) -> get_pi_vcpu_info() -> "vcpu_info->pi_desc_addr
= __sme_set(page_to_phys((*svm)->avic_backing_page));" -> crash! or is
there anything which make this impossible?

The patch is an improvement anyway, so
Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly