linux-kernel - Re: [PATCH] KVM: ensure APICv is considered inactive if there is no APIC

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Date:   Tue, 30 Nov 2021 13:05:59 +0000
From:   Ignat Korchagin <ignat@...udflare.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: ensure APICv is considered inactive if there is no APIC

On Tue, Nov 30, 2021 at 12:37 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> kvm_vcpu_apicv_active() returns false if a virtual machine has no in-kernel
> local APIC, however kvm_apicv_activated might still be true if there are
> no reasons to disable APICv; in fact it is quite likely that there is none
> because APICv is inhibited by specific configurations of the local APIC
> and those configurations cannot be programmed.  This triggers a WARN:
>
>    WARN_ON_ONCE(kvm_apicv_activated(vcpu->kvm) != kvm_vcpu_apicv_active(vcpu));
>
> To avoid this, introduce another cause for APICv inhibition, namely the
> absence of an in-kernel local APIC.  This cause is enabled by default,
> and is dropped by either KVM_CREATE_IRQCHIP or the enabling of
> KVM_CAP_IRQCHIP_SPLIT.
>
> Reported-by: Ignat Korchagin <ignat@...udflare.com>
> Fixes: ee49a8932971 ("KVM: x86: Move SVM's APICv sanity check to common x86", 2021-10-22)
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 1 +
>  arch/x86/kvm/svm/avic.c         | 1 +
>  arch/x86/kvm/vmx/vmx.c          | 1 +
>  arch/x86/kvm/x86.c              | 9 +++++----
>  4 files changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6ac61f85e07b..860ed500580c 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1036,6 +1036,7 @@ struct kvm_x86_msr_filter {
>  #define APICV_INHIBIT_REASON_PIT_REINJ  4
>  #define APICV_INHIBIT_REASON_X2APIC    5
>  #define APICV_INHIBIT_REASON_BLOCKIRQ  6
> +#define APICV_INHIBIT_REASON_ABSENT    7
>
>  struct kvm_arch {
>         unsigned long n_used_mmu_pages;
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index affc0ea98d30..5a55a78e2f50 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -900,6 +900,7 @@ int svm_update_pi_irte(struct kvm *kvm, unsigned int host_irq,
>  bool svm_check_apicv_inhibit_reasons(ulong bit)
>  {
>         ulong supported = BIT(APICV_INHIBIT_REASON_DISABLE) |
> +                         BIT(APICV_INHIBIT_REASON_ABSENT) |
>                           BIT(APICV_INHIBIT_REASON_HYPERV) |
>                           BIT(APICV_INHIBIT_REASON_NESTED) |
>                           BIT(APICV_INHIBIT_REASON_IRQWIN) |
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1fadec8cbf96..ca1fd93c1dc9 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7525,6 +7525,7 @@ static void hardware_unsetup(void)
>  static bool vmx_check_apicv_inhibit_reasons(ulong bit)
>  {
>         ulong supported = BIT(APICV_INHIBIT_REASON_DISABLE) |
> +                         BIT(APICV_INHIBIT_REASON_ABSENT) |
>                           BIT(APICV_INHIBIT_REASON_HYPERV) |
>                           BIT(APICV_INHIBIT_REASON_BLOCKIRQ);
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0ee1a039b490..e0aa4dd53c7f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5740,6 +5740,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>                 smp_wmb();
>                 kvm->arch.irqchip_mode = KVM_IRQCHIP_SPLIT;
>                 kvm->arch.nr_reserved_ioapic_pins = cap->args[0];
> +               kvm_request_apicv_update(kvm, true, APICV_INHIBIT_REASON_ABSENT);
>                 r = 0;
>  split_irqchip_unlock:
>                 mutex_unlock(&kvm->lock);
> @@ -6120,6 +6121,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
>                 /* Write kvm->irq_routing before enabling irqchip_in_kernel. */
>                 smp_wmb();
>                 kvm->arch.irqchip_mode = KVM_IRQCHIP_KERNEL;
> +               kvm_request_apicv_update(kvm, true, APICV_INHIBIT_REASON_ABSENT);
>         create_irqchip_unlock:
>                 mutex_unlock(&kvm->lock);
>                 break;
> @@ -8818,10 +8820,9 @@ static void kvm_apicv_init(struct kvm *kvm)
>  {
>         init_rwsem(&kvm->arch.apicv_update_lock);
>
> -       if (enable_apicv)
> -               clear_bit(APICV_INHIBIT_REASON_DISABLE,
> -                         &kvm->arch.apicv_inhibit_reasons);
> -       else
> +       set_bit(APICV_INHIBIT_REASON_ABSENT,
> +               &kvm->arch.apicv_inhibit_reasons);
> +       if (!enable_apicv)
>                 set_bit(APICV_INHIBIT_REASON_DISABLE,
>                         &kvm->arch.apicv_inhibit_reasons);
>  }
> --
> 2.31.1
>

Tested-by: Ignat Korchagin <ignat@...udflare.com>