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

Ignat Korchagin
2021-11-30T13:06:14Z

On Tue, Nov 30, 2021 at 12:37 PM Paolo Bonzini <pbonzini@redhat.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@cloudflare.com>
> Fixes: ee49a8932971 ("KVM: x86: Move SVM's APICv sanity check to common x86", 2021-10-22)
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.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@cloudflare.com>