[PATCH v4 1/2] virtio-mem: Correct default THP size for ARM64

[Gavin Shan]

The default block size is same as to the THP size, which is either
retrieved from "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
or hardcoded to 2MB. There are flaws in both mechanisms and this
intends to fix them up.

  * When "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size" is
    used to getting the THP size, 32MB and 512MB are valid values
    when we have 16KB and 64KB page size on ARM64.

  * When the hardcoded THP size is used, 2MB, 32MB and 512MB are
    valid values when we have 4KB, 16KB and 64KB page sizes on
    ARM64.

Co-developed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 hw/virtio/virtio-mem.c | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index 04c223b0c9..1ca45336dc 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -46,14 +46,25 @@
  */
 #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
+static uint32_t virtio_mem_default_thp_size(void)
+{
+    uint32_t default_thp_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
+
+#if defined(__x86_64__) || defined(__arm__) || defined(__powerpc64__)
+    default_thp_size = 2 * MiB;
+#elif defined(__aarch64__)
+    if (qemu_real_host_page_size == 4 * KiB) {
+        default_thp_size = 2 * MiB;
+    } else if (qemu_real_host_page_size == 16 * KiB) {
+        default_thp_size = 32 * MiB;
+    } else if (qemu_real_host_page_size == 64 * KiB) {
+        default_thp_size = 512 * MiB;
+    }
 #endif
 
+    return default_thp_size;
+}
+
 /*
  * We want to have a reasonable default block size such that
  * 1. We avoid splitting THPs when unplugging memory, which degrades
@@ -86,11 +97,8 @@ static uint32_t virtio_mem_thp_size(void)
     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) {
+        /* Sanity-check the value and fallback to something reasonable. */
+        if (!tmp || !is_power_of_2(tmp)) {
             warn_report("Read unsupported THP size: %" PRIx64, tmp);
         } else {
             thp_size = tmp;
@@ -98,7 +106,7 @@ static uint32_t virtio_mem_thp_size(void)
     }
 
     if (!thp_size) {
-        thp_size = VIRTIO_MEM_DEFAULT_THP_SIZE;
+        thp_size = virtio_mem_default_thp_size();
         warn_report("Could not detect THP size, falling back to %" PRIx64
                     "  MiB.", thp_size / MiB);
     }
-- 
2.23.0