linux-kernel - Re: [v3 13/26] KVM: Define a new interface kvm_find_dest

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Message-ID: <20150109145435.GA22469@potion.brq.redhat.com>
Date:	Fri, 9 Jan 2015 15:54:36 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Feng Wu <feng.wu@...el.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, gleb@...nel.org, pbonzini@...hat.com,
	dwmw2@...radead.org, joro@...tes.org, alex.williamson@...hat.com,
	jiang.liu@...ux.intel.com, eric.auger@...aro.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	kvm@...r.kernel.org
Subject: Re: [v3 13/26] KVM: Define a new interface kvm_find_dest_vcpu() for
 VT-d PI

2014-12-12 23:14+0800, Feng Wu:
> This patch defines a new interface kvm_find_dest_vcpu for
> VT-d PI, which can returns the destination vCPU of the
> interrupt for guests.
> 
> Since VT-d PI cannot handle broadcast/multicast interrupt,
> Here we only handle Fixed and Lowest priority interrupts.
> 
> The current method of handling guest lowest priority interrtups
> is to use a counter 'apic_arb_prio' for each vCPU, we choose the
> vCPU with smallest 'apic_arb_prio' and then increase it by 1.
> However, for VT-d PI, we cannot re-use this, since we no longer
> have control to 'apic_arb_prio' with posted interrupt direct
> delivery by Hardware.
> 
> Here, we introduce a similar way with 'apic_arb_prio' to handle
> guest lowest priority interrtups when VT-d PI is used. Here is the
> ideas:
> - Each vCPU has a counter 'round_robin_counter'.
> - When guests sets an interrupts to lowest priority, we choose
> the vCPU with smallest 'round_robin_counter' as the destination,
> then increase it.

There are two points relevant to this patch in new KVM's implementation,
("KVM: x86: amend APIC lowest priority arbitration",
 https://lkml.org/lkml/2015/1/9/362)

1) lowest priority depends on TPR
2) there is no need for balancing

(1) has to be considered with PI as well.
I kept (2) to avoid whining from people building on that behaviour, but
lowest priority backed by PI could be transparent without it.

Patch below removes the balancing, but I am not sure this is a price we
allowed ourselves to pay ... what are your opinions?


---8<---
KVM: x86: don't balance lowest priority interrupts

Balancing is not mandated by specification and real hardware most likely
doesn't do it.  We break backward compatibility to allow optimizations.
(Posted interrupts can deliver to only one fixed destination.)

Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
---
 arch/x86/include/asm/kvm_host.h | 1 -
 arch/x86/kvm/lapic.c            | 8 ++------
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 97a5dd0222c8..aa4bd8286232 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -370,7 +370,6 @@ struct kvm_vcpu_arch {
 	u64 apic_base;
 	struct kvm_lapic *apic;    /* kernel irqchip context */
 	unsigned long apic_attention;
-	int32_t apic_arb_prio;
 	int mp_state;
 	u64 ia32_misc_enable_msr;
 	bool tpr_access_reporting;
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 5b9d8c589bba..eb85af8e8fc0 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -749,7 +749,6 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
 				  trig_mode, vector);
 	switch (delivery_mode) {
 	case APIC_DM_LOWEST:
-		vcpu->arch.apic_arb_prio++;
 	case APIC_DM_FIXED:
 		/* FIXME add logic for vcpu on reset */
 		if (unlikely(!apic_enabled(apic)))
@@ -837,11 +836,9 @@ int kvm_apic_compare_prio(struct kvm_vcpu *vcpu1, struct kvm_vcpu *vcpu2)
 	 *      - uses the APR register (which also considers ISR and IRR),
 	 *      - chooses the highest APIC ID when APRs are identical,
 	 *      - and allows a focus processor.
-	 * XXX: pseudo-balancing with apic_arb_prio is a KVM-specific feature
 	 */
-	int tpr = kvm_apic_get_reg(vcpu1->arch.apic, APIC_TASKPRI) -
-	          kvm_apic_get_reg(vcpu2->arch.apic, APIC_TASKPRI);
-	return tpr ? : vcpu1->arch.apic_arb_prio - vcpu2->arch.apic_arb_prio;
+	return kvm_apic_get_reg(vcpu1->arch.apic, APIC_TASKPRI) -
+	       kvm_apic_get_reg(vcpu2->arch.apic, APIC_TASKPRI);
 }
 
 static void kvm_ioapic_send_eoi(struct kvm_lapic *apic, int vector)
@@ -1595,7 +1592,6 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
 	vcpu->arch.pv_eoi.msr_val = 0;
 	apic_update_ppr(apic);
 
-	vcpu->arch.apic_arb_prio = 0;
 	vcpu->arch.apic_attention = 0;
 
 	apic_debug("%s: vcpu=%p, id=%d, base_msr="
-- 
2.2.0

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