When we intercept a DIAG_9C from the guest we verify that the
target real CPU associated with the virtual CPU designated by
the guest is running and if not we forward the DIAG_9C to the
target real CPU.
To avoid a diag9c storm we allow a maximal rate of diag9c forwarding.
The rate is calculated as a count per second defined as a new
parameter of the s390 kvm module: diag9c_forwarding_hz .
The default value of 0 is to not forward diag9c.
Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
---
Documentation/virt/kvm/s390-diag.rst | 33 ++++++++++++++++++++++++++++
arch/s390/include/asm/kvm_host.h | 1 +
arch/s390/include/asm/smp.h | 1 +
arch/s390/kernel/smp.c | 1 +
arch/s390/kvm/diag.c | 31 +++++++++++++++++++++++---
arch/s390/kvm/kvm-s390.c | 6 +++++
arch/s390/kvm/kvm-s390.h | 8 +++++++
7 files changed, 78 insertions(+), 3 deletions(-)
diff --git a/Documentation/virt/kvm/s390-diag.rst b/Documentation/virt/kvm/s390-diag.rst
index eaac4864d3d6..ca85f030eb0b 100644
--- a/Documentation/virt/kvm/s390-diag.rst
+++ b/Documentation/virt/kvm/s390-diag.rst
@@ -84,3 +84,36 @@ If the function code specifies 0x501, breakpoint functions may be performed.
This function code is handled by userspace.
This diagnose function code has no subfunctions and uses no parameters.
+
+
+DIAGNOSE function code 'X'9C - Voluntary Time Slice Yield
+---------------------------------------------------------
+
+General register 1 contains the target CPU address.
+
+In a guest of a hypervisor like LPAR, KVM or z/VM using shared host CPUs,
+DIAGNOSE with function code 0x9c may improve system performance by
+yielding the host CPU on which the guest CPU is running to be assigned
+to another guest CPU, preferably the logical CPU containing the specified
+target CPU.
+
+
+DIAG 'X'9C forwarding
++++++++++++++++++++++
+
+The guest may send a DIAGNOSE 0x9c in order to yield to a certain
+other vcpu. An example is a Linux guest that tries to yield to the vcpu
+that is currently holding a spinlock, but not running.
+
+However, on the host the real cpu backing the vcpu may itself not be
+running.
+Forwarding the DIAGNOSE 0x9c initially sent by the guest to yield to
+the backing cpu will hopefully cause that cpu, and thus subsequently
+the guest's vcpu, to be scheduled.
+
+
+diag9c_forwarding_hz
+ KVM kernel parameter allowing to specify the maximum number of DIAGNOSE
+ 0x9c forwarding per second in the purpose of avoiding a DIAGNOSE 0x9c
+ forwarding storm.
+ A value of 0 turns the forwarding off.
diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index 527776a1f076..cb19508c22fb 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -456,6 +456,7 @@ struct kvm_vcpu_stat {
u64 diagnose_44;
u64 diagnose_9c;
u64 diagnose_9c_ignored;
+ u64 diagnose_9c_forward;
u64 diagnose_258;
u64 diagnose_308;
u64 diagnose_500;
diff --git a/arch/s390/include/asm/smp.h b/arch/s390/include/asm/smp.h
index 01e360004481..e317fd4866c1 100644
--- a/arch/s390/include/asm/smp.h
+++ b/arch/s390/include/asm/smp.h
@@ -63,5 +63,6 @@ extern void __noreturn cpu_die(void);
extern void __cpu_die(unsigned int cpu);
extern int __cpu_disable(void);
extern void schedule_mcck_handler(void);
+void notrace smp_yield_cpu(int cpu);
#endif /* __ASM_SMP_H */
diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 27c763014114..cecdecd33a58 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -422,6 +422,7 @@ void notrace smp_yield_cpu(int cpu)
asm volatile("diag %0,0,0x9c"
: : "d" (pcpu_devices[cpu].address));
}
+EXPORT_SYMBOL_GPL(smp_yield_cpu);
/*
* Send cpus emergency shutdown signal. This gives the cpus the
diff --git a/arch/s390/kvm/diag.c b/arch/s390/kvm/diag.c
index 5b8ec1c447e1..02c146f9e5cd 100644
--- a/arch/s390/kvm/diag.c
+++ b/arch/s390/kvm/diag.c
@@ -150,6 +150,19 @@ static int __diag_time_slice_end(struct kvm_vcpu *vcpu)
return 0;
}
+static int forward_cnt;
+static unsigned long cur_slice;
+
+static int diag9c_forwarding_overrun(void)
+{
+ /* Reset the count on a new slice */
+ if (time_after(jiffies, cur_slice)) {
+ cur_slice = jiffies;
+ forward_cnt = diag9c_forwarding_hz / HZ;
+ }
+ return forward_cnt-- <= 0 ? 1 : 0;
+}
+
static int __diag_time_slice_end_directed(struct kvm_vcpu *vcpu)
{
struct kvm_vcpu *tcpu;
@@ -167,9 +180,21 @@ static int __diag_time_slice_end_directed(struct kvm_vcpu *vcpu)
if (!tcpu)
goto no_yield;
- /* target already running */
- if (READ_ONCE(tcpu->cpu) >= 0)
- goto no_yield;
+ /* target guest VCPU already running */
+ if (READ_ONCE(tcpu->cpu) >= 0) {
+ if (!diag9c_forwarding_hz || diag9c_forwarding_overrun())
+ goto no_yield;
+
+ /* target host CPU already running */
+ if (!vcpu_is_preempted(tcpu->cpu))
+ goto no_yield;
+ smp_yield_cpu(tcpu->cpu);
+ VCPU_EVENT(vcpu, 5,
+ "diag time slice end directed to %d: yield forwarded",
+ tid);
+ vcpu->stat.diagnose_9c_forward++;
+ return 0;
+ }
if (kvm_vcpu_yield_to(tcpu) <= 0)
goto no_yield;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 053ef36784e4..7b113d14ed7c 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -157,6 +157,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
VCPU_STAT("instruction_diag_44", diagnose_44),
VCPU_STAT("instruction_diag_9c", diagnose_9c),
VCPU_STAT("diag_9c_ignored", diagnose_9c_ignored),
+ VCPU_STAT("diag_9c_forward", diagnose_9c_forward),
VCPU_STAT("instruction_diag_258", diagnose_258),
VCPU_STAT("instruction_diag_308", diagnose_308),
VCPU_STAT("instruction_diag_500", diagnose_500),
@@ -190,6 +191,11 @@ static bool use_gisa = true;
module_param(use_gisa, bool, 0644);
MODULE_PARM_DESC(use_gisa, "Use the GISA if the host supports it.");
+/* maximum diag9c forwarding per second */
+unsigned int diag9c_forwarding_hz;
+module_param(diag9c_forwarding_hz, uint, 0644);
+MODULE_PARM_DESC(diag9c_forwarding_hz, "Maximum diag9c forwarding per second, 0 to turn off");
+
/*
* For now we handle at most 16 double words as this is what the s390 base
* kernel handles and stores in the prefix page. If we ever need to go beyond
diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
index 79dcd647b378..9fad25109b0d 100644
--- a/arch/s390/kvm/kvm-s390.h
+++ b/arch/s390/kvm/kvm-s390.h
@@ -471,4 +471,12 @@ void kvm_s390_reinject_machine_check(struct kvm_vcpu *vcpu,
* @kvm: the KVM guest
*/
void kvm_s390_vcpu_crypto_reset_all(struct kvm *kvm);
+
+/**
+ * diag9c_forwarding_hz
+ *
+ * Set the maximum number of diag9c forwarding per second
+ */
+extern unsigned int diag9c_forwarding_hz;
+
#endif
--
2.17.1
