[Qemu-devel] [PATCH 2/2] s390x/kvm: passing max memory size to accelerator

[Jens Freimann <jfrei@linux.vnet.ibm.com>]

From: Dominik Dingel <dingel@linux.vnet.ibm.com>

With "KVM: s390: Allow userspace to limit guest memory size" KVM is able to
do some optimizations based on the guest memory limit.

The guest memory limit is computed by the initial definition and with the notion of
hotplugged memory.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Guenther Hutzl <hutzl@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
---
 hw/s390x/s390-virtio-ccw.c | 10 ++++++++++
 target-s390x/cpu.h         | 14 ++++++++++++++
 target-s390x/kvm.c         | 45 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 078371a..eea0742 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -97,6 +97,7 @@ static void ccw_init(MachineState *machine)
     ram_addr_t pad_size = 0;
     ram_addr_t maxmem = qemu_opt_get_size(opts, "maxmem", my_ram_size);
     ram_addr_t standby_mem_size = maxmem - my_ram_size;
+    uint64_t kvm_limit;
 
     /* The storage increment size is a multiple of 1M and is a power of 2.
      * The number of storage increments must be MAX_STORAGE_INCREMENTS or fewer.
@@ -121,6 +122,15 @@ static void ccw_init(MachineState *machine)
 
     /* let's propagate the changed ram size into the global variable. */
     ram_size = my_ram_size;
+    machine->maxram_size = my_ram_size + standby_mem_size;
+
+    ret = s390_set_memory_limit(machine->maxram_size, &kvm_limit);
+    if (ret == -E2BIG) {
+        hw_error("qemu: host supports a maximum of %" PRIu64 " GB",
+                 kvm_limit >> 30);
+    } else if (ret) {
+        hw_error("qemu: setting the guest size failed");
+    }
 
     /* get a BUS */
     css_bus = virtual_css_bus_init();
diff --git a/target-s390x/cpu.h b/target-s390x/cpu.h
index be53b5a..f89e1e3 100644
--- a/target-s390x/cpu.h
+++ b/target-s390x/cpu.h
@@ -1062,6 +1062,7 @@ int kvm_s390_get_memslot_count(KVMState *s);
 void kvm_s390_clear_cmma_callback(void *opaque);
 int kvm_s390_set_cpu_state(S390CPU *cpu, uint8_t cpu_state);
 void kvm_s390_reset_vcpu(S390CPU *cpu);
+int kvm_s390_set_mem_limit(KVMState *s, uint64_t new_limit, uint64_t *hw_limit);
 #else
 static inline void kvm_s390_io_interrupt(uint16_t subchannel_id,
                                         uint16_t subchannel_nr,
@@ -1099,8 +1100,21 @@ static inline int kvm_s390_set_cpu_state(S390CPU *cpu, uint8_t cpu_state)
 static inline void kvm_s390_reset_vcpu(S390CPU *cpu)
 {
 }
+static inline int kvm_s390_set_mem_limit(KVMState *s, uint64_t new_limit,
+                                         uint64_t *hw_limit)
+{
+    return 0;
+}
 #endif
 
+static inline int s390_set_memory_limit(uint64_t new_limit, uint64_t *hw_limit)
+{
+    if (kvm_enabled()) {
+        return kvm_s390_set_mem_limit(kvm_state, new_limit, hw_limit);
+    }
+    return 0;
+}
+
 static inline void cmma_reset(S390CPU *cpu)
 {
     if (kvm_enabled()) {
diff --git a/target-s390x/kvm.c b/target-s390x/kvm.c
index 6ef714c..f4bcca1 100644
--- a/target-s390x/kvm.c
+++ b/target-s390x/kvm.c
@@ -122,6 +122,51 @@ static int cap_async_pf;
 
 static void *legacy_s390_alloc(size_t size, uint64_t *align);
 
+static int kvm_s390_supports_mem_limit(KVMState *s)
+{
+    struct kvm_device_attr attr = {
+        .group = KVM_S390_VM_MEM_CTRL,
+        .attr = KVM_S390_VM_MEM_LIMIT_SIZE,
+    };
+
+    return (kvm_vm_ioctl(s, KVM_HAS_DEVICE_ATTR, &attr) == 0);
+}
+
+static int kvm_s390_query_mem_limit(KVMState *s, uint64_t *memory_limit)
+{
+    struct kvm_device_attr attr = {
+        .group = KVM_S390_VM_MEM_CTRL,
+        .attr = KVM_S390_VM_MEM_LIMIT_SIZE,
+        .addr = (uint64_t) memory_limit,
+    };
+
+    return kvm_vm_ioctl(s, KVM_GET_DEVICE_ATTR, &attr);
+}
+
+int kvm_s390_set_mem_limit(KVMState *s, uint64_t new_limit, uint64_t *hw_limit)
+{
+    int rc;
+
+    struct kvm_device_attr attr = {
+        .group = KVM_S390_VM_MEM_CTRL,
+        .attr = KVM_S390_VM_MEM_LIMIT_SIZE,
+        .addr = (uint64_t) &new_limit,
+    };
+
+    if (!kvm_s390_supports_mem_limit(s)) {
+        return 0;
+    }
+
+    rc = kvm_s390_query_mem_limit(s, hw_limit);
+    if (rc) {
+        return rc;
+    } else if (*hw_limit < new_limit) {
+        return -E2BIG;
+    }
+
+    return kvm_vm_ioctl(s, KVM_SET_DEVICE_ATTR, &attr);
+}
+
 static int kvm_s390_check_clear_cmma(KVMState *s)
 {
     struct kvm_device_attr attr = {
-- 
2.1.4