util/mmap-alloc: Pass flags instead of separate bools to qemu_ram_mmap()

Let's pass flags instead of bools to prepare for passing other flags and
update the documentation of qemu_ram_mmap(). Introduce new QEMU_MAP_
flags that abstract the mmap() PROT_ and MAP_ flag handling and simplify
it.

We expose only flags that are currently supported by qemu_ram_mmap().
Maybe, we'll see qemu_mmap() in the future as well that can implement these
flags.

Note: We don't use MAP_ flags as some flags (e.g., MAP_SYNC) are only
defined for some systems and we want to always be able to identify
these flags reliably inside qemu_ram_mmap() -- for example, to properly
warn when some future flags are not available or effective on a system.
Also, this way we can simplify PROT_ handling as well.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-8-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/mmap-alloc.h

@@ -7,18 +7,22 @@ size_t qemu_fd_getpagesize(int fd);
 size_t qemu_mempath_getpagesize(const char *mem_path);
 
 /**
- * qemu_ram_mmap: mmap the specified file or device.
+ * qemu_ram_mmap: mmap anonymous memory, the specified file or device.
+ *
+ * mmap() abstraction to map guest RAM, simplifying flag handling, taking
+ * care of alignment requirements and installing guard pages.
  *
  * Parameters:
  *  @fd: the file or the device to mmap
  *  @size: the number of bytes to be mmaped
  *  @align: if not zero, specify the alignment of the starting mapping address;
  *          otherwise, the alignment in use will be determined by QEMU.
- *  @readonly: true for a read-only mapping, false for read/write.
- *  @shared: map has RAM_SHARED flag.
- *  @is_pmem: map has RAM_PMEM flag.
+ *  @qemu_map_flags: QEMU_MAP_* flags
  *  @map_offset: map starts at offset of map_offset from the start of fd
  *
+ * Internally, MAP_PRIVATE, MAP_ANONYMOUS and MAP_SHARED_VALIDATE are set
+ * implicitly based on other parameters.
+ *
  * Return:
  *  On success, return a pointer to the mapped area.
  *  On failure, return MAP_FAILED.
@@ -26,9 +30,7 @@ size_t qemu_mempath_getpagesize(const char *mem_path);
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
-                    bool readonly,
-                    bool shared,
-                    bool is_pmem,
+                    uint32_t qemu_map_flags,
                     off_t map_offset);
 
 void qemu_ram_munmap(int fd, void *ptr, size_t size);

include/qemu/osdep.h

@@ -366,6 +366,24 @@ void *qemu_anon_ram_alloc(size_t size, uint64_t *align, bool shared);
 void qemu_vfree(void *ptr);
 void qemu_anon_ram_free(void *ptr, size_t size);
 
+/*
+ * Abstraction of PROT_ and MAP_ flags as passed to mmap(), for example,
+ * consumed by qemu_ram_mmap().
+ */
+
+/* Map PROT_READ instead of PROT_READ | PROT_WRITE. */
+#define QEMU_MAP_READONLY   (1 << 0)
+
+/* Use MAP_SHARED instead of MAP_PRIVATE. */
+#define QEMU_MAP_SHARED     (1 << 1)
+
+/*
+ * Use MAP_SYNC | MAP_SHARED_VALIDATE if supported. Ignored without
+ * QEMU_MAP_SHARED. If mapping fails, warn and fallback to !QEMU_MAP_SYNC.
+ */
+#define QEMU_MAP_SYNC       (1 << 2)
+
+
 #define QEMU_MADV_INVALID -1
 
 #if defined(CONFIG_MADVISE)

softmmu/physmem.c

@@ -1540,6 +1540,7 @@ static void *file_ram_alloc(RAMBlock *block,
                             off_t offset,
                             Error **errp)
 {
+    uint32_t qemu_map_flags;
     void *area;
 
     block->page_size = qemu_fd_getpagesize(fd);
@@ -1587,9 +1588,10 @@ static void *file_ram_alloc(RAMBlock *block,
         perror("ftruncate");
     }
 
-    area = qemu_ram_mmap(fd, memory, block->mr->align, readonly,
-                         block->flags & RAM_SHARED, block->flags & RAM_PMEM,
-                         offset);
+    qemu_map_flags = readonly ? QEMU_MAP_READONLY : 0;
+    qemu_map_flags |= (block->flags & RAM_SHARED) ? QEMU_MAP_SHARED : 0;
+    qemu_map_flags |= (block->flags & RAM_PMEM) ? QEMU_MAP_SYNC : 0;
+    area = qemu_ram_mmap(fd, memory, block->mr->align, qemu_map_flags, offset);
     if (area == MAP_FAILED) {
         error_setg_errno(errp, errno,
                          "unable to map backing store for guest RAM");

util/mmap-alloc.c

@@ -118,9 +118,12 @@ static void *mmap_reserve(size_t size, int fd)
  * Activate memory in a reserved region from the given fd (if any), to make
  * it accessible.
  */
-static void *mmap_activate(void *ptr, size_t size, int fd, bool readonly,
-                           bool shared, bool is_pmem, off_t map_offset)
+static void *mmap_activate(void *ptr, size_t size, int fd,
+                           uint32_t qemu_map_flags, off_t map_offset)
 {
+    const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
+    const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
+    const bool sync = qemu_map_flags & QEMU_MAP_SYNC;
     const int prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
     int map_sync_flags = 0;
     int flags = MAP_FIXED;
@@ -128,7 +131,7 @@ static void *mmap_activate(void *ptr, size_t size, int fd, bool readonly,
 
     flags |= fd == -1 ? MAP_ANONYMOUS : 0;
     flags |= shared ? MAP_SHARED : MAP_PRIVATE;
-    if (shared && is_pmem) {
+    if (shared && sync) {
         map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
     }
 
@@ -173,9 +176,7 @@ static inline size_t mmap_guard_pagesize(int fd)
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
-                    bool readonly,
-                    bool shared,
-                    bool is_pmem,
+                    uint32_t qemu_map_flags,
                     off_t map_offset)
 {
     const size_t guard_pagesize = mmap_guard_pagesize(fd);
@@ -199,7 +200,7 @@ void *qemu_ram_mmap(int fd,
 
     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 
-    ptr = mmap_activate(guardptr + offset, size, fd, readonly, shared, is_pmem,
+    ptr = mmap_activate(guardptr + offset, size, fd, qemu_map_flags,
                         map_offset);
     if (ptr == MAP_FAILED) {
         munmap(guardptr, total);

util/oslib-posix.c

@@ -229,8 +229,9 @@ void *qemu_memalign(size_t alignment, size_t size)
 /* alloc shared memory pages */
 void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared)
 {
+    const uint32_t qemu_map_flags = shared ? QEMU_MAP_SHARED : 0;
     size_t align = QEMU_VMALLOC_ALIGN;
-    void *ptr = qemu_ram_mmap(-1, size, align, false, shared, false, 0);
+    void *ptr = qemu_ram_mmap(-1, size, align, qemu_map_flags, 0);
 
     if (ptr == MAP_FAILED) {
         return NULL;