[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>

5 files changed, 34 insertions, 6 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
 #
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)
diff --git a/include/asm-s390/kvm_host.h b/include/asm-s390/kvm_host.h
index f8204a4f2e02..18cbd8a39796 100644
--- a/include/asm-s390/kvm_host.h
+++ b/include/asm-s390/kvm_host.h
@@ -104,6 +104,7 @@ struct sie_block {
 
 struct kvm_vcpu_stat {
         u32 exit_userspace;
+        u32 exit_null;
         u32 exit_external_request;
         u32 exit_external_interrupt;
         u32 exit_stop_request;