[PATCH v3 03/70] RAMBlock/guest_memfd: Enable KVM_GUEST_MEMFD_ALLOW_HUGEPAGE

[Xiaoyao Li <xiaoyao.li@intel.com>]

KVM allows KVM_GUEST_MEMFD_ALLOW_HUGEPAGE for guest memfd. When the
flag is set, KVM tries to allocate memory with transparent hugeapge at
first and falls back to non-hugepage on failure.

However, KVM defines one restriction that size must be hugepage size
aligned when KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is set.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
v3:
 - New one in v3.
---
 system/physmem.c | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/system/physmem.c b/system/physmem.c
index 0af2213cbd9c..c56b17e44df6 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -1803,6 +1803,40 @@ static void dirty_memory_extend(ram_addr_t old_ram_size,
     }
 }
 
+#ifdef CONFIG_KVM
+#define HPAGE_PMD_SIZE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
+#define DEFAULT_PMD_SIZE (1ul << 21)
+
+static uint32_t get_thp_size(void)
+{
+    gchar *content = NULL;
+    const char *endptr;
+    static uint64_t thp_size = 0;
+    uint64_t tmp;
+
+    if (thp_size != 0) {
+        return thp_size;
+    }
+
+    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 and fallback to something reasonable. */
+        if (!tmp || !is_power_of_2(tmp)) {
+            warn_report("Read unsupported THP size: %" PRIx64, tmp);
+        } else {
+            thp_size = tmp;
+        }
+    }
+
+    if (!thp_size) {
+        thp_size = DEFAULT_PMD_SIZE;
+    }
+
+    return thp_size;
+}
+#endif
+
 static void ram_block_add(RAMBlock *new_block, Error **errp)
 {
     const bool noreserve = qemu_ram_is_noreserve(new_block);
@@ -1844,8 +1878,8 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
 #ifdef CONFIG_KVM
     if (kvm_enabled() && new_block->flags & RAM_GUEST_MEMFD &&
         new_block->guest_memfd < 0) {
-        /* TODO: to decide if KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is supported */
-        uint64_t flags = 0;
+        uint64_t flags = QEMU_IS_ALIGNED(new_block->max_length, get_thp_size()) ?
+                         KVM_GUEST_MEMFD_ALLOW_HUGEPAGE : 0;
         new_block->guest_memfd = kvm_create_guest_memfd(new_block->max_length,
                                                         flags, errp);
         if (new_block->guest_memfd < 0) {
-- 
2.34.1