linux-kernel - [PATCH RFC 1/6] KVM: fix guest_mode optimization in kvm_make_all_cpus

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Message-Id: <20170406202056.18379-2-rkrcmar@redhat.com>
Date:   Thu,  6 Apr 2017 22:20:51 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Christoffer Dall <cdall@...aro.org>,
        Andrew Jones <drjones@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: [PATCH RFC 1/6] KVM: fix guest_mode optimization in kvm_make_all_cpus_request()

We have kvm_arch_vcpu_should_kick() to decide whether the target cpu
needs to be kicked.  The previous condition was wrong, because
architectures that don't use vcpu->mode would not get interrupts and
also suboptimal, because it sent IPI in cases where none was necessary.

The situation is even more convoluted.  MIPS and POWERPC return 1 from
kvm_arch_vcpu_should_kick(), but implement vcpu->mode for some reason,
so now they might kick uselessly.  This is not a huge problem.

s390, on the other hand, never changed vcpu->mode, so it would always be
OUTSIDE_GUEST_MODE before and therefore didn't send IPIs.
I don't see a reason why s390 had kvm_make_all_cpus_request() that did
nothing but set the bit in vcpu->request, so the new behavior seems
better.

Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
---
 arch/s390/kvm/kvm-s390.c | 4 +---
 virt/kvm/kvm_main.c      | 2 +-
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index fd6cd05bb6a7..45b6d9ca5d24 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2130,9 +2130,7 @@ static void kvm_gmap_notifier(struct gmap *gmap, unsigned long start,
 
 int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
 {
-	/* kvm common code refers to this, but never calls it */
-	BUG();
-	return 0;
+	return 1;
 }
 
 static int kvm_arch_vcpu_ioctl_get_one_reg(struct kvm_vcpu *vcpu,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f489167839c4..2389e9c41cd2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -187,7 +187,7 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
 		smp_mb__after_atomic();
 
 		if (cpus != NULL && cpu != -1 && cpu != me &&
-		      kvm_vcpu_exiting_guest_mode(vcpu) != OUTSIDE_GUEST_MODE)
+				kvm_arch_vcpu_should_kick(vcpu))
 			cpumask_set_cpu(cpu, cpus);
 	}
 	if (unlikely(cpus == NULL))
-- 
2.12.0