util: exclude mmap-alloc.c from compilation target on Emscripten

Emscripten does not support partial unmapping of mmapped memory
regions[1]. This limitation prevents correct implementation of qemu_ram_mmap
and qemu_ram_munmap, which rely on partial unmap behavior.

As a workaround, this commit excludes mmap-alloc.c from the Emscripten
build. Instead, for Emscripten build, this modifies qemu_anon_ram_alloc to
use qemu_memalign in place of qemu_ram_mmap, and disable memory backends
that rely on mmap, such as memory-backend-file and memory-backend-shm.

[1] https://github.com/emscripten-core/emscripten/blob/d4a74336f23214bf3304d9eb0d03966786b30a36/system/lib/libc/emscripten_mmap.c#L61

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Link: https://lore.kernel.org/r/76834f933ee4f14eeb5289d21c59d306886e58e9.1745820062.git.ktokunaga.mail@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index a697c602c6..4ff577e5de 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -58,6 +58,7 @@
 #include <lwp.h>
 #endif
 
+#include "qemu/memalign.h"
 #include "qemu/mmap-alloc.h"
 
 #define MAX_MEM_PREALLOC_THREAD_COUNT 16
@@ -210,11 +211,21 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared,
     const uint32_t qemu_map_flags = (shared ? QEMU_MAP_SHARED : 0) |
                                     (noreserve ? QEMU_MAP_NORESERVE : 0);
     size_t align = QEMU_VMALLOC_ALIGN;
+#ifndef EMSCRIPTEN
     void *ptr = qemu_ram_mmap(-1, size, align, qemu_map_flags, 0);
 
     if (ptr == MAP_FAILED) {
         return NULL;
     }
+#else
+    /*
+     * qemu_ram_mmap is not implemented for Emscripten. Use qemu_memalign
+     * for the anonymous allocation. noreserve is ignored as there is no swap
+     * space on Emscripten, and shared is ignored as there is no other
+     * processes on Emscripten.
+     */
+    void *ptr = qemu_memalign(align, size);
+#endif
 
     if (alignment) {
         *alignment = align;
@@ -227,7 +238,16 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared,
 void qemu_anon_ram_free(void *ptr, size_t size)
 {
     trace_qemu_anon_ram_free(ptr, size);
+#ifndef EMSCRIPTEN
     qemu_ram_munmap(-1, ptr, size);
+#else
+    /*
+     * qemu_ram_munmap is not implemented for Emscripten and qemu_memalign
+     * was used for the allocation. Use the corresponding freeing function
+     * here.
+     */
+    qemu_vfree(ptr);
+#endif
 }
 
 void qemu_socket_set_block(int fd)
@@ -588,7 +608,15 @@ bool qemu_prealloc_mem(int fd, char *area, size_t sz, int max_threads,
 {
     static gsize initialized;
     int ret;
+#ifndef EMSCRIPTEN
     size_t hpagesize = qemu_fd_getpagesize(fd);
+#else
+    /*
+     * mmap-alloc.c is excluded from Emscripten build, so qemu_fd_getpagesize
+     * is unavailable. Fallback to the lower level implementation.
+     */
+    size_t hpagesize = qemu_real_host_page_size();
+#endif
     size_t numpages = DIV_ROUND_UP(sz, hpagesize);
     bool use_madv_populate_write;
     struct sigaction act;