linux-kernel - [PATCH 2/3] KVM: X86: Narrow down the IPI fastpath to single target IPI

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Date:   Thu, 26 Mar 2020 10:20:01 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH 2/3] KVM: X86: Narrow down the IPI fastpath to single target IPI

From: Wanpeng Li <wanpengli@...cent.com>

The original single target IPI fastpath patch forgot to filter the 
ICR destination shorthand field. Multicast IPI is not suitable for 
this feature since wakeup the multiple sleeping vCPUs will extend 
the interrupt disabled time, it especially worse in the over-subscribe 
and VM has a little bit more vCPUs scenario. Let's narrow it down to 
single target IPI.

Two VMs, each is 76 vCPUs, one running 'ebizzy -M', the other 
running cyclictest on all vCPUs, w/ this patch, the avg score 
of cyclictest can improve more than 5%. (pv tlb, pv ipi, pv 
sched yield are disabled during testing to avoid the disturb).

Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
 arch/x86/kvm/x86.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9232b15..50ef1c5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1554,7 +1554,10 @@ EXPORT_SYMBOL_GPL(kvm_emulate_wrmsr);
  */
 static int handle_fastpath_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data)
 {
-	if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic) &&
+	if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic))
+		return 1;
+
+	if (((data & APIC_SHORT_MASK) == APIC_DEST_NOSHORT) &&
 		((data & APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
 		((data & APIC_MODE_MASK) == APIC_DM_FIXED)) {

-- 
2.7.4