arm/kvm: Enable support for KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE

Now that we have Eager Page Split support added for ARM in the kernel,
enable it in Qemu. This adds,
 -eager-split-size to -accel sub-options to set the eager page split chunk size.
 -enable KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE.

The chunk size specifies how many pages to break at a time, using a
single allocation. Bigger the chunk size, more pages need to be
allocated ahead of time.

Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Message-id: 20230905091246.1931-1-shameerali.kolothum.thodi@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

accel/kvm/kvm-all.c

@@ -3763,6 +3763,7 @@ static void kvm_accel_instance_init(Object *obj)
     /* KVM dirty ring is by default off */
     s->kvm_dirty_ring_size = 0;
     s->kvm_dirty_ring_with_bitmap = false;
+    s->kvm_eager_split_size = 0;
     s->notify_vmexit = NOTIFY_VMEXIT_OPTION_RUN;
     s->notify_window = 0;
     s->xen_version = 0;

include/sysemu/kvm_int.h

@@ -116,6 +116,7 @@ struct KVMState
     uint64_t kvm_dirty_ring_bytes;  /* Size of the per-vcpu dirty ring */
     uint32_t kvm_dirty_ring_size;   /* Number of dirty GFNs per ring */
     bool kvm_dirty_ring_with_bitmap;
+    uint64_t kvm_eager_split_size;  /* Eager Page Splitting chunk size */
     struct KVMDirtyRingReaper reaper;
     NotifyVmexitOption notify_vmexit;
     uint32_t notify_window;

qemu-options.hx

@@ -186,6 +186,7 @@ DEF("accel", HAS_ARG, QEMU_OPTION_accel,
     "                split-wx=on|off (enable TCG split w^x mapping)\n"
     "                tb-size=n (TCG translation block cache size)\n"
     "                dirty-ring-size=n (KVM dirty ring GFN count, default 0)\n"
+    "                eager-split-size=n (KVM Eager Page Split chunk size, default 0, disabled. ARM only)\n"
     "                notify-vmexit=run|internal-error|disable,notify-window=n (enable notify VM exit and set notify window, x86 only)\n"
     "                thread=single|multi (enable multi-threaded TCG)\n", QEMU_ARCH_ALL)
 SRST
@@ -244,6 +245,20 @@ SRST
         is disabled (dirty-ring-size=0).  When enabled, KVM will instead
         record dirty pages in a bitmap.
 
+    ``eager-split-size=n``
+        KVM implements dirty page logging at the PAGE_SIZE granularity and
+        enabling dirty-logging on a huge-page requires breaking it into
+        PAGE_SIZE pages in the first place. KVM on ARM does this splitting
+        lazily by default. There are performance benefits in doing huge-page
+        split eagerly, especially in situations where TLBI costs associated
+        with break-before-make sequences are considerable and also if guest
+        workloads are read intensive. The size here specifies how many pages
+        to break at a time and needs to be a valid block size which is
+        1GB/2MB/4KB, 32MB/16KB and 512MB/64KB for 4KB/16KB/64KB PAGE_SIZE
+        respectively. Be wary of specifying a higher size as it will have an
+        impact on the memory. By default, this feature is disabled
+        (eager-split-size=0).
+
     ``notify-vmexit=run|internal-error|disable,notify-window=n``
         Enables or disables notify VM exit support on x86 host and specify
         the corresponding notify window to trigger the VM exit if enabled.

target/arm/kvm.c

@@ -30,6 +30,7 @@
 #include "exec/address-spaces.h"
 #include "hw/boards.h"
 #include "hw/irq.h"
+#include "qapi/visitor.h"
 #include "qemu/log.h"
 
 const KVMCapabilityInfo kvm_arch_required_capabilities[] = {
@@ -287,6 +288,26 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
         }
     }
 
+    if (s->kvm_eager_split_size) {
+        uint32_t sizes;
+
+        sizes = kvm_vm_check_extension(s, KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES);
+        if (!sizes) {
+            s->kvm_eager_split_size = 0;
+            warn_report("Eager Page Split support not available");
+        } else if (!(s->kvm_eager_split_size & sizes)) {
+            error_report("Eager Page Split requested chunk size not valid");
+            ret = -EINVAL;
+        } else {
+            ret = kvm_vm_enable_cap(s, KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE, 0,
+                                    s->kvm_eager_split_size);
+            if (ret < 0) {
+                error_report("Enabling of Eager Page Split failed: %s",
+                             strerror(-ret));
+            }
+        }
+    }
+
     kvm_arm_init_debug(s);
 
     return ret;
@@ -1069,6 +1090,46 @@ bool kvm_arch_cpu_check_are_resettable(void)
     return true;
 }
 
+static void kvm_arch_get_eager_split_size(Object *obj, Visitor *v,
+                                          const char *name, void *opaque,
+                                          Error **errp)
+{
+    KVMState *s = KVM_STATE(obj);
+    uint64_t value = s->kvm_eager_split_size;
+
+    visit_type_size(v, name, &value, errp);
+}
+
+static void kvm_arch_set_eager_split_size(Object *obj, Visitor *v,
+                                          const char *name, void *opaque,
+                                          Error **errp)
+{
+    KVMState *s = KVM_STATE(obj);
+    uint64_t value;
+
+    if (s->fd != -1) {
+        error_setg(errp, "Unable to set early-split-size after KVM has been initialized");
+        return;
+    }
+
+    if (!visit_type_size(v, name, &value, errp)) {
+        return;
+    }
+
+    if (value && !is_power_of_2(value)) {
+        error_setg(errp, "early-split-size must be a power of two");
+        return;
+    }
+
+    s->kvm_eager_split_size = value;
+}
+
 void kvm_arch_accel_class_init(ObjectClass *oc)
 {
+    object_class_property_add(oc, "eager-split-size", "size",
+                              kvm_arch_get_eager_split_size,
+                              kvm_arch_set_eager_split_size, NULL, NULL);
+
+    object_class_property_set_description(oc, "eager-split-size",
+        "Eager Page Split chunk size for hugepages. (default: 0, disabled)");
 }