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] d4a74336f2/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>

backends/meson.build

@@ -12,8 +12,10 @@ system_ss.add([files(
 
 if host_os != 'windows'
   system_ss.add(files('rng-random.c'))
-  system_ss.add(files('hostmem-file.c'))
-  system_ss.add([files('hostmem-shm.c'), rt])
+  if host_os != 'emscripten'
+    system_ss.add(files('hostmem-file.c'))
+    system_ss.add([files('hostmem-shm.c'), rt])
+  endif
 endif
 if host_os == 'linux'
   system_ss.add(files('hostmem-memfd.c'))

system/memory.c

@@ -1627,7 +1627,7 @@ bool memory_region_init_resizeable_ram(MemoryRegion *mr,
     return true;
 }
 
-#ifdef CONFIG_POSIX
+#if defined(CONFIG_POSIX) && !defined(EMSCRIPTEN)
 bool memory_region_init_ram_from_file(MemoryRegion *mr,
                                       Object *owner,
                                       const char *name,

system/physmem.c

@@ -1243,7 +1243,7 @@ long qemu_maxrampagesize(void)
     return pagesize;
 }
 
-#ifdef CONFIG_POSIX
+#if defined(CONFIG_POSIX) && !defined(EMSCRIPTEN)
 static int64_t get_file_size(int fd)
 {
     int64_t size;
@@ -1978,7 +1978,7 @@ out_free:
     }
 }
 
-#ifdef CONFIG_POSIX
+#if defined(CONFIG_POSIX) && !defined(EMSCRIPTEN)
 RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, ram_addr_t max_size,
                                  qemu_ram_resize_cb resized, MemoryRegion *mr,
                                  uint32_t ram_flags, int fd, off_t offset,
@@ -2158,7 +2158,8 @@ RAMBlock *qemu_ram_alloc_internal(ram_addr_t size, ram_addr_t max_size,
     assert(!host ^ (ram_flags & RAM_PREALLOC));
     assert(max_size >= size);
 
-#ifdef CONFIG_POSIX         /* ignore RAM_SHARED for Windows */
+    /* ignore RAM_SHARED for Windows and emscripten*/
+#if defined(CONFIG_POSIX) && !defined(EMSCRIPTEN)
     if (!host) {
         if (!share_flags && current_machine->aux_ram_share) {
             ram_flags |= RAM_SHARED;
@@ -2255,7 +2256,7 @@ static void reclaim_ramblock(RAMBlock *block)
         ;
     } else if (xen_enabled()) {
         xen_invalidate_map_cache_entry(block->host);
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(EMSCRIPTEN)
     } else if (block->fd >= 0) {
         qemu_ram_munmap(block->fd, block->host, block->max_length);
         close(block->fd);

util/meson.build

@@ -11,7 +11,9 @@ if host_os != 'windows'
   endif
   util_ss.add(files('compatfd.c'))
   util_ss.add(files('event_notifier-posix.c'))
-  util_ss.add(files('mmap-alloc.c'))
+  if host_os != 'emscripten'
+    util_ss.add(files('mmap-alloc.c'))
+  endif
   freebsd_dep = []
   if host_os == 'freebsd'
     freebsd_dep = util

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;