[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>

1 files changed, 29 insertions, 1 deletions

diff --git a/arch/s390/kernel/entry64.S b/arch/s390/kernel/entry64.S
index a57909d63149..fee10177dbfc 100644
--- a/arch/s390/kernel/entry64.S
+++ b/arch/s390/kernel/entry64.S
@@ -607,14 +607,37 @@ io_restore_trace_psw:
 #endif
 
 #
-# switch to kernel stack, then check TIF bits
+# There is work todo, we need to check if we return to userspace, then
+# check, if we are in SIE, if yes leave it
 #
 io_work:
         tm      SP_PSW+1(%r15),0x01     # returning to user ?
 #ifndef CONFIG_PREEMPT
+#if defined(CONFIG_KVM) || defined(CONFIG_KVM_MODULE)
+        jnz     io_work_user            # yes -> no need to check for SIE
+        la      %r1, BASED(sie_opcode)  # we return to kernel here
+        lg      %r2, SP_PSW+8(%r15)
+        clc     0(2,%r1), 0(%r2)        # is current instruction = SIE?
+        jne     io_restore              # no-> return to kernel
+        lg      %r1, SP_PSW+8(%r15)     # yes-> add 4 bytes to leave SIE
+        aghi    %r1, 4
+        stg     %r1, SP_PSW+8(%r15)
+        j       io_restore              # return to kernel
+#else
         jno     io_restore              # no-> skip resched & signal
+#endif
 #else
         jnz     io_work_user            # yes -> do resched & signal
+#if defined(CONFIG_KVM) || defined(CONFIG_KVM_MODULE)
+        la      %r1, BASED(sie_opcode)
+        lg      %r2, SP_PSW+8(%r15)
+        clc     0(2,%r1), 0(%r2)        # is current instruction = SIE?
+        jne     0f                      # no -> leave PSW alone
+        lg      %r1, SP_PSW+8(%r15)     # yes-> add 4 bytes to leave SIE
+        aghi    %r1, 4
+        stg     %r1, SP_PSW+8(%r15)
+0:
+#endif
         # check for preemptive scheduling
         icm     %r0,15,__TI_precount(%r9)
         jnz     io_restore              # preemption is disabled
@@ -652,6 +675,11 @@ io_work_loop:
         j       io_restore
 io_work_done:
 
+#if defined(CONFIG_KVM) || defined(CONFIG_KVM_MODULE)
+sie_opcode:
+        .long 0xb2140000
+#endif
+
 #
 # _TIF_MCCK_PENDING is set, call handler
 #