[Qemu-devel] [PATCH] migration: initialize RAM to zero

[Paolo Bonzini <pbonzini@redhat.com>]

Using qemu_memalign only leaves the RAM zero by chance, because libc
will usually use mmap to satisfy our huge requests.  But memory will
not be zero when using MALLOC_PERTURB_ with a nonzero value.  In the
case of incoming migration, this breaks a recently-introduced
invariant (commit f1c7279, migration: do not sent zero pages in
bulk stage, 2013-03-26).

To fix this, use mmap ourselves to get a well-aligned, always zero
block for the RAM.  Mmap-ed memory is easy to "trim" at the sides.

This also removes the need to do something special on valgrind
(see commit c2a8238a, Support running QEMU on Valgrind, 2011-10-31).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 util/oslib-posix.c | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 433dd68..91f5aab 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -52,12 +52,8 @@ extern int daemon(int, int);
 #include "sysemu/sysemu.h"
 #include "trace.h"
 #include "qemu/sockets.h"
+#include <sys/mman.h>
 
-#if defined(CONFIG_VALGRIND)
-static int running_on_valgrind = -1;
-#else
-#  define running_on_valgrind 0
-#endif
 #ifdef CONFIG_LINUX
 #include <sys/syscall.h>
 #endif
@@ -108,22 +104,22 @@ void *qemu_memalign(size_t alignment, size_t size)
 /* alloc shared memory pages */
 void *qemu_vmalloc(size_t size)
 {
-    void *ptr;
     size_t align = QEMU_VMALLOC_ALIGN;
+    size_t total = size + align - getpagesize();
+    void *ptr = mmap(0, total, PROT_READ | PROT_WRITE,
+                     MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
 
-#if defined(CONFIG_VALGRIND)
-    if (running_on_valgrind < 0) {
-        /* First call, test whether we are running on Valgrind.
-           This is a substitute for RUNNING_ON_VALGRIND from valgrind.h. */
-        const char *ld = getenv("LD_PRELOAD");
-        running_on_valgrind = (ld != NULL && strstr(ld, "vgpreload"));
-    }
-#endif
+    ptr += offset;
+    total -= offset;
 
-    if (size < align || running_on_valgrind) {
-        align = getpagesize();
+    if (offset > 0) {
+        munmap(ptr - offset, offset);
+    }
+    if (total > size) {
+        munmap(ptr + size, total - size);
     }
-    ptr = qemu_memalign(align, size);
+
     trace_qemu_vmalloc(size, ptr);
     return ptr;
 }
-- 
1.8.1.4