[PATCH/RFC 2/3] s390/kvm: Platform specific kvm_arch_vcpu_dont_yield

[Christian Borntraeger <borntraeger@de.ibm.com>]

From: Michael Mueller <mimu@linux.vnet.ibm.com>

Commit "s390/kvm: Use common waitqueue" caused a performance regression
on s390. It turned out that a yield candidate was missed by just a simple
test on its non-empty waitqueue. If an interrupt is outstanding, the candidate
might be suitable. kvm_arch_vcpu_dont_yield is extended by a test that
additionally tests for not yet delivered interrupts.

Significant performance measurement work and code analysis to solve
this issue was provided by Mao Chuan Li and his team in Beijing.

Signed-off-by: Michael Mueller <mimu@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 arch/s390/kvm/Kconfig    | 1 +
 arch/s390/kvm/kvm-s390.c | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/arch/s390/kvm/Kconfig b/arch/s390/kvm/Kconfig
index c8bacbc..e44adef 100644
--- a/arch/s390/kvm/Kconfig
+++ b/arch/s390/kvm/Kconfig
@@ -25,6 +25,7 @@ config KVM
 	select HAVE_KVM_EVENTFD
 	select KVM_ASYNC_PF
 	select KVM_ASYNC_PF_SYNC
+	select HAVE_KVM_ARCH_VCPU_DONT_YIELD
 	---help---
 	  Support hosting paravirtualized guest machines using the SIE
 	  virtualization capability on the mainframe. This should work
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index a5da2cc..1a33e1e 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1231,6 +1231,13 @@ int kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
 	return VM_FAULT_SIGBUS;
 }
 
+#ifdef CONFIG_HAVE_KVM_ARCH_VCPU_DONT_YIELD
+bool kvm_arch_vcpu_dont_yield(struct kvm_vcpu *vcpu)
+{
+	return waitqueue_active(&vcpu->wq) && !kvm_cpu_has_interrupt(vcpu);
+}
+#endif
+
 void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *free,
 			   struct kvm_memory_slot *dont)
 {
-- 
1.8.4.2