[S390] s390-kvm: leave sie context on work. Removes preemption requirement

From: Martin Schwidefsky <schwidefsky@de.ibm.com>

This patch fixes a bug with cpu bound guest on kvm-s390. Sometimes it
was impossible to deliver a signal to a spinning guest. We used
preemption as a circumvention. The preemption notifiers called
vcpu_load, which checked for pending signals and triggered a host
intercept. But even with preemption, a sigkill was not delivered
immediately.

This patch changes the low level host interrupt handler to check for the
SIE  instruction, if TIF_WORK is set. In that case we change the
instruction pointer of the return PSW to rerun the vcpu_run loop. The kvm
code sees an intercept reason 0 if that happens. This patch adds accounting
for these types of intercept as well.

The advantages:
- works with and without preemption
- signals are delivered immediately
- much better host latencies without preemption

Acked-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

3 files changed, 4 insertions, 5 deletions

diff --git a/arch/s390/kvm/Kconfig b/arch/s390/kvm/Kconfig
index 1761b74d639b..e051cad1f1e0 100644
--- a/arch/s390/kvm/Kconfig
+++ b/arch/s390/kvm/Kconfig
@@ -22,7 +22,6 @@ config KVM
 	select PREEMPT_NOTIFIERS
 	select ANON_INODES
 	select S390_SWITCH_AMODE
-	select PREEMPT
 	---help---
 	  Support hosting paravirtualized guest machines using the SIE
 	  virtualization capability on the mainframe. This should work
diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
index 349581a26103..47a0b642174c 100644
--- a/arch/s390/kvm/intercept.c
+++ b/arch/s390/kvm/intercept.c
@@ -105,6 +105,9 @@ static intercept_handler_t instruction_handlers[256] = {
 static int handle_noop(struct kvm_vcpu *vcpu)
 {
 	switch (vcpu->arch.sie_block->icptcode) {
+	case 0x0:
+		vcpu->stat.exit_null++;
+		break;
 	case 0x10:
 		vcpu->stat.exit_external_request++;
 		break;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 98d1e73e01f1..0ac36a649eba 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -31,6 +31,7 @@
 
 struct kvm_stats_debugfs_item debugfs_entries[] = {
 	{ "userspace_handled", VCPU_STAT(exit_userspace) },
+	{ "exit_null", VCPU_STAT(exit_null) },
 	{ "exit_validity", VCPU_STAT(exit_validity) },
 	{ "exit_stop_request", VCPU_STAT(exit_stop_request) },
 	{ "exit_external_request", VCPU_STAT(exit_external_request) },
@@ -221,10 +222,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	vcpu->arch.guest_fpregs.fpc &= FPC_VALID_MASK;
 	restore_fp_regs(&vcpu->arch.guest_fpregs);
 	restore_access_regs(vcpu->arch.guest_acrs);
-
-	if (signal_pending(current))
-		atomic_set_mask(CPUSTAT_STOP_INT,
-			&vcpu->arch.sie_block->cpuflags);
 }
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)