virtio-mem: Probe THP size to determine default block size

Let's allow a minimum block size of 1 MiB in all configurations. Select
the default block size based on
- The page size of the memory backend.
- The THP size if the memory backend size corresponds to the real host
  page size.
- The global minimum of 1 MiB.
and warn if something smaller is configured by the user.

VIRTIO_MEM only supports Linux (depends on LINUX), so we can probe the
THP size unconditionally.

For now we only support virtio-mem on x86-64 - there isn't a user-visible
change (x86-64 only supports 2 MiB THP on the PMD level) - the default
was, and will be 2 MiB.

If we ever have THP on the PUD level (e.g., 1 GiB THP on x86-64), we
expect it to be more transparent - e.g., to only optimize fully populated
ranges unless explicitly told /configured otherwise (in contrast to PMD
THP).

Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20201008083029.9504-4-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-mem.c

@@ -33,10 +33,83 @@
 #include "trace.h"
 
 /*
- * Use QEMU_VMALLOC_ALIGN, so no THP will have to be split when unplugging
+ * Let's not allow blocks smaller than 1 MiB, for example, to keep the tracking
- * memory (e.g., 2MB on x86_64).
+ * bitmap small.
  */
-#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)QEMU_VMALLOC_ALIGN)
+#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)(1 * MiB))
+
+#if defined(__x86_64__) || defined(__arm__) || defined(__aarch64__) || \
+    defined(__powerpc64__)
+#define VIRTIO_MEM_DEFAULT_THP_SIZE ((uint32_t)(2 * MiB))
+#else
+        /* fallback to 1 MiB (e.g., the THP size on s390x) */
+#define VIRTIO_MEM_DEFAULT_THP_SIZE VIRTIO_MEM_MIN_BLOCK_SIZE
+#endif
+
+/*
+ * We want to have a reasonable default block size such that
+ * 1. We avoid splitting THPs when unplugging memory, which degrades
+ *    performance.
+ * 2. We avoid placing THPs for plugged blocks that also cover unplugged
+ *    blocks.
+ *
+ * The actual THP size might differ between Linux kernels, so we try to probe
+ * it. In the future (if we ever run into issues regarding 2.), we might want
+ * to disable THP in case we fail to properly probe the THP size, or if the
+ * block size is configured smaller than the THP size.
+ */
+static uint32_t thp_size;
+
+#define HPAGE_PMD_SIZE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
+static uint32_t virtio_mem_thp_size(void)
+{
+    gchar *content = NULL;
+    const char *endptr;
+    uint64_t tmp;
+
+    if (thp_size) {
+        return thp_size;
+    }
+
+    /*
+     * Try to probe the actual THP size, fallback to (sane but eventually
+     * incorrect) default sizes.
+     */
+    if (g_file_get_contents(HPAGE_PMD_SIZE_PATH, &content, NULL, NULL) &&
+        !qemu_strtou64(content, &endptr, 0, &tmp) &&
+        (!endptr || *endptr == '\n')) {
+        /*
+         * Sanity-check the value, if it's too big (e.g., aarch64 with 64k base
+         * pages) or weird, fallback to something smaller.
+         */
+        if (!tmp || !is_power_of_2(tmp) || tmp > 16 * MiB) {
+            warn_report("Read unsupported THP size: %" PRIx64, tmp);
+        } else {
+            thp_size = tmp;
+        }
+    }
+
+    if (!thp_size) {
+        thp_size = VIRTIO_MEM_DEFAULT_THP_SIZE;
+        warn_report("Could not detect THP size, falling back to %" PRIx64
+                    "  MiB.", thp_size / MiB);
+    }
+
+    g_free(content);
+    return thp_size;
+}
+
+static uint64_t virtio_mem_default_block_size(RAMBlock *rb)
+{
+    const uint64_t page_size = qemu_ram_pagesize(rb);
+
+    /* We can have hugetlbfs with a page size smaller than the THP size. */
+    if (page_size == qemu_real_host_page_size) {
+        return MAX(page_size, virtio_mem_thp_size());
+    }
+    return MAX(page_size, VIRTIO_MEM_MIN_BLOCK_SIZE);
+}
+
 /*
  * Size the usable region bigger than the requested size if possible. Esp.
  * Linux guests will only add (aligned) memory blocks in case they fully
@@ -443,10 +516,23 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
     rb = vmem->memdev->mr.ram_block;
     page_size = qemu_ram_pagesize(rb);
 
+    /*
+     * If the block size wasn't configured by the user, use a sane default. This
+     * allows using hugetlbfs backends of any page size without manual
+     * intervention.
+     */
+    if (!vmem->block_size) {
+        vmem->block_size = virtio_mem_default_block_size(rb);
+    }
+
     if (vmem->block_size < page_size) {
         error_setg(errp, "'%s' property has to be at least the page size (0x%"
                    PRIx64 ")", VIRTIO_MEM_BLOCK_SIZE_PROP, page_size);
         return;
+    } else if (vmem->block_size < virtio_mem_default_block_size(rb)) {
+        warn_report("'%s' property is smaller than the default block size (%"
+                    PRIx64 " MiB)", VIRTIO_MEM_BLOCK_SIZE_PROP,
+                    virtio_mem_default_block_size(rb) / MiB);
     } else if (!QEMU_IS_ALIGNED(vmem->requested_size, vmem->block_size)) {
         error_setg(errp, "'%s' property has to be multiples of '%s' (0x%" PRIx64
                    ")", VIRTIO_MEM_REQUESTED_SIZE_PROP,
@@ -742,6 +828,18 @@ static void virtio_mem_get_block_size(Object *obj, Visitor *v, const char *name,
     const VirtIOMEM *vmem = VIRTIO_MEM(obj);
     uint64_t value = vmem->block_size;
 
+    /*
+     * If not configured by the user (and we're not realized yet), use the
+     * default block size we would use with the current memory backend.
+     */
+    if (!value) {
+        if (vmem->memdev && memory_region_is_ram(&vmem->memdev->mr)) {
+            value = virtio_mem_default_block_size(vmem->memdev->mr.ram_block);
+        } else {
+            value = virtio_mem_thp_size();
+        }
+    }
+
     visit_type_size(v, name, &value, errp);
 }
 
@@ -821,7 +919,6 @@ static void virtio_mem_instance_init(Object *obj)
 {
     VirtIOMEM *vmem = VIRTIO_MEM(obj);
 
-    vmem->block_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
     notifier_list_init(&vmem->size_change_notifiers);
     vmem->precopy_notifier.notify = virtio_mem_precopy_notify;