linux-kernel - Re: [PATCH v3] KVM: x86: inhibit APICv when KVM_GUESTDBG

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Message-ID: <YYQz3E/eNxdnNwBj@google.com>
Date:   Thu, 4 Nov 2021 19:26:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v3] KVM: x86: inhibit APICv when KVM_GUESTDBG_BLOCKIRQ
 active

On Wed, Nov 03, 2021, Maxim Levitsky wrote:
> KVM_GUESTDBG_BLOCKIRQ relies on interrupts being injected using
> standard kvm's inject_pending_event, and not via APICv/AVIC.
> 
> Since this is a debug feature, just inhibit APICv/AVIC while
> KVM_GUESTDBG_BLOCKIRQ is in use on at least one vCPU.

Very clever!

> Fixes: 61e5f69ef0837 ("KVM: x86: implement KVM_GUESTDBG_BLOCKIRQ")
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---

With the below nits resolved (tested on Intel w/ APICv):

Reviewed-and-tested-by: Sean Christopherson <seanjc@...gle.com>

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ac83d873d65b0..5d30cea58182e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10703,6 +10703,25 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
>  	return ret;
>  }
>  
> +static void kvm_arch_vcpu_guestdbg_update_apicv_inhibit(struct kvm *kvm)
> +{
> +	struct kvm_vcpu *vcpu = NULL;

vcpu doesn't need to be initialized.

> +	int i;

Nit, I'd prefer we use reverse fir tree when it's convenient, i.e.

	bool block_irq_used = false;
	struct kvm_vcpu *vcpu;
	int i;

> +	bool block_irq_used = false;
> +
> +	down_write(&kvm->arch.apicv_update_lock);
> +
> +	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		if (vcpu->guest_debug & KVM_GUESTDBG_BLOCKIRQ) {
> +			block_irq_used = true;
> +			break;
> +		}
> +	}
> +	__kvm_request_apicv_update(kvm, !block_irq_used,
> +					       APICV_INHIBIT_REASON_BLOCKIRQ);

Heh, this indentation is still messed up, I think you need to change your

	if (r == -ENOCOFFEE)
		maxim_get_coffee();

to

	while (r == -ENOCOFFEE)
		r = maxim_get_coffee();

> +	up_write(&kvm->arch.apicv_update_lock);
> +}
> +
>  int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  					struct kvm_guest_debug *dbg)
>  {
> @@ -10755,6 +10774,8 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>  
>  	static_call(kvm_x86_update_exception_bitmap)(vcpu);
>  
> +	kvm_arch_vcpu_guestdbg_update_apicv_inhibit(vcpu->kvm);
> +
>  	r = 0;
>  
>  out:
> -- 
> 2.26.3
>