util: add qemu_get_host_physmem utility function

This will be used in a future patch. For POSIX systems _SC_PHYS_PAGES
isn't standardised but at least appears in the man pages for
Open/FreeBSD. The result is advisory so any users of it shouldn't just
fail if we can't work it out.

The win32 stub currently returns 0 until someone with a Windows system
can develop and test a patch.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Cc: BALATON Zoltan <balaton@eik.bme.hu>
Cc: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Message-Id: <20200724064509.331-5-alex.bennee@linaro.org>

include/qemu/osdep.h

@@ -668,4 +668,16 @@ static inline void qemu_reset_optind(void)
  */
 char *qemu_get_host_name(Error **errp);
 
+/**
+ * qemu_get_host_physmem:
+ *
+ * Operating system agnostic way of querying host memory.
+ *
+ * Returns amount of physical memory on the system. This is purely
+ * advisery and may return 0 if we can't work it out. At the other
+ * end we saturate to SIZE_MAX if you are lucky enough to have that
+ * much memory.
+ */
+size_t qemu_get_host_physmem(void);
+
 #endif

util/oslib-posix.c

@@ -841,3 +841,18 @@ char *qemu_get_host_name(Error **errp)
 
     return g_steal_pointer(&hostname);
 }
+
+size_t qemu_get_host_physmem(void)
+{
+#ifdef _SC_PHYS_PAGES
+    long pages = sysconf(_SC_PHYS_PAGES);
+    if (pages > 0) {
+        if (pages > SIZE_MAX / qemu_real_host_page_size) {
+            return SIZE_MAX;
+        } else {
+            return pages * qemu_real_host_page_size;
+        }
+    }
+#endif
+    return 0;
+}

util/oslib-win32.c

@@ -828,3 +828,9 @@ char *qemu_get_host_name(Error **errp)
 
     return g_utf16_to_utf8(tmp, size, NULL, NULL, NULL);
 }
+
+size_t qemu_get_host_physmem(void)
+{
+    /* currently unimplemented */
+    return 0;
+}