linux-kernel - Re: [PATCH v5 32/33] KVM: x86: Allow APICv APIC ID inhibit to be cleared

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Date:   Sun, 08 Jan 2023 17:25:11 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>,
        Greg Edwards <gedwards@....com>
Subject: Re: [PATCH v5 32/33] KVM: x86: Allow APICv APIC ID inhibit to be
 cleared

On Fri, 2023-01-06 at 01:13 +0000, Sean Christopherson wrote:
> From: Greg Edwards <gedwards@....com>
> 
> Legacy kernels prior to commit 4399c03c6780 ("x86/apic: Remove
> verify_local_APIC()") write the APIC ID of the boot CPU twice to verify
> a functioning local APIC.  This results in APIC acceleration inhibited
> on these kernels for reason APICV_INHIBIT_REASON_APIC_ID_MODIFIED.
> 
> Allow the APICV_INHIBIT_REASON_APIC_ID_MODIFIED inhibit reason to be
> cleared if/when all APICs in xAPIC mode set their APIC ID back to the
> expected vcpu_id value.
> 
> Fold the functionality previously in kvm_lapic_xapic_id_updated() into
> kvm_recalculate_apic_map(), as this allows examining all APICs in one
> pass.
> 
> Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
> Signed-off-by: Greg Edwards <gedwards@....com>
> Link: https://lore.kernel.org/r/20221117183247.94314-1-gedwards@ddn.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/lapic.c | 41 +++++++++++++++--------------------------
>  1 file changed, 15 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 0faf80cdc1be..856e81a2dc64 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -236,6 +236,7 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>  	struct kvm_vcpu *vcpu;
>  	unsigned long i;
>  	u32 max_id = 255; /* enough space for any xAPIC ID */
> +	bool xapic_id_mismatch = false;
>  
>  	/* Read kvm->arch.apic_map_dirty before kvm->arch.apic_map.  */
>  	if (atomic_read_acquire(&kvm->arch.apic_map_dirty) == CLEAN)
> @@ -285,6 +286,15 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>  		xapic_id = kvm_xapic_id(apic);
>  		x2apic_id = kvm_x2apic_id(apic);
>  
> +		/*
> +		 * Deliberately truncate the vCPU ID when detecting a mismatched
> +		 * APIC ID to avoid false positives if the vCPU ID, i.e. x2APIC
> +		 * ID, is a 32-bit value.  Any unwanted aliasing due to
> +		 * truncation results will be detected below.
> +		 */
> +		if (!apic_x2apic_mode(apic) && xapic_id != (u8)vcpu->vcpu_id)
> +			xapic_id_mismatch = true;
> +
>  		/*
>  		 * Apply KVM's hotplug hack if userspace has enable 32-bit APIC
>  		 * IDs.  Allow sending events to vCPUs by their x2APIC ID even
> @@ -401,6 +411,11 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>  	else
>  		kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED);
>  
> +	if (xapic_id_mismatch)
> +		kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
> +	else
> +		kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
> +
>  	old = rcu_dereference_protected(kvm->arch.apic_map,
>  			lockdep_is_held(&kvm->arch.apic_map_lock));
>  	rcu_assign_pointer(kvm->arch.apic_map, new);
> @@ -2160,28 +2175,6 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
>  	}
>  }
>  
> -static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
> -{
> -	struct kvm *kvm = apic->vcpu->kvm;
> -
> -	if (!kvm_apic_hw_enabled(apic))
> -		return;
> -
> -	if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm))
> -		return;
> -
> -	/*
> -	 * Deliberately truncate the vCPU ID when detecting a modified APIC ID
> -	 * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit
> -	 * value.  If the wrap/truncation results in unwanted aliasing, APICv
> -	 * will be inhibited as part of updating KVM's optimized APIC maps.
> -	 */
> -	if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id)
> -		return;
> -
> -	kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);
> -}
> -
>  static int get_lvt_index(u32 reg)
>  {
>  	if (reg == APIC_LVTCMCI)
> @@ -2202,7 +2195,6 @@ static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
>  	case APIC_ID:		/* Local APIC ID */
>  		if (!apic_x2apic_mode(apic)) {
>  			kvm_apic_set_xapic_id(apic, val >> 24);
> -			kvm_lapic_xapic_id_updated(apic);
>  		} else {
>  			ret = 1;
>  		}
> @@ -2923,9 +2915,6 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>  	}
>  	memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
>  
> -	if (!apic_x2apic_mode(apic))
> -		kvm_lapic_xapic_id_updated(apic);
> -
>  	atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
>  	kvm_recalculate_apic_map(vcpu->kvm);
>  	kvm_apic_set_version(vcpu);

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky