boot-qemu.py: Refactor getting values from configuration

This will come in handy in trying to warn people when they are missing
configurations needed for certain features.

While we are at it, make getting the configuration value a little more
robust using re.search() + match groups.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

boot-qemu.py

@@ -154,6 +154,42 @@ def can_use_kvm(can_test_for_kvm, guest_arch):
     return False
 
 
+def get_config_val(kernel_arg, config_key):
+    """
+    Attempt to get configuration value from a .config relative to
+    kernel_location.
+
+    Parameters:
+        kernel_arg (str): The value of the '--kernel' argument.
+
+    Returns:
+        The configuration value if it can be found, None if not.
+    """
+    # kernel_arg is either a path to the kernel source or a full kernel
+    # location. If it is a file, we need to strip off the basename so that we
+    # can easily navigate around with '..'.
+    if (kernel_dir := Path(kernel_arg)).is_file():
+        kernel_dir = kernel_dir.parent
+
+    # If kernel_location is the kernel source, the configuration will be at
+    # <kernel_dir>/.config
+    #
+    # If kernel_location is a full kernel location, it could either be:
+    #   * <kernel_dir>/.config (if the image is vmlinux)
+    #   * <kernel_dir>/../../../.config (if the image is in arch/*/boot/)
+    #   * <kernel_dir>/config (if the image is in a TuxMake folder)
+    config_locations = [".config", "../../../.config", "config"]
+    if (config_file := utils.find_first_file(kernel_dir,
+                                             config_locations,
+                                             required=False)):
+        config_txt = config_file.read_text(encoding='utf-8')
+        if (match := re.search(f"^{config_key}=(.*)$", config_txt,
+                               flags=re.M)):
+            return match.groups()[0]
+
+    return None
+
+
 def get_smp_value(args):
     """
     Get the value of '-smp' based on user input and kernel configuration.
@@ -178,41 +214,16 @@ def get_smp_value(args):
     if args.smp:
         return args.smp
 
-    # kernel_location is either a path to the kernel source or a full kernel
-    # location. If it is a file, we need to strip off the basename so that we
-    # can easily navigate around with '..'.
-    kernel_dir = Path(args.kernel_location)
-    if kernel_dir.is_file():
-        kernel_dir = kernel_dir.parent
-
-    # If kernel_location is the kernel source, the configuration will be at
-    # <kernel_dir>/.config
-    #
-    # If kernel_location is a full kernel location, it could either be:
-    #   * <kernel_dir>/.config (if the image is vmlinux)
-    #   * <kernel_dir>/../../../.config (if the image is in arch/*/boot/)
-    #   * <kernel_dir>/config (if the image is in a TuxMake folder)
-    config_file = None
-    for config_name in [".config", "../../../.config", "config"]:
-        config_path = kernel_dir.joinpath(config_name)
-        if config_path.is_file():
-            config_file = config_path
-            break
-
     # Choose a sensible default value based on treewide defaults for
     # CONFIG_NR_CPUS then get the actual value if possible.
-    config_nr_cpus = 8
-    if config_file:
-        with open(config_file, encoding='utf-8') as file:
-            for line in file:
-                if "CONFIG_NR_CPUS=" in line:
-                    config_nr_cpus = int(line.split("=", 1)[1])
-                    break
+    if not (config_nr_cpus := get_config_val(args.kernel_location,
+                                             'CONFIG_NR_CPUS')):
+        config_nr_cpus = 8
 
     # Use the minimum of the number of usable processors for the script or
     # CONFIG_NR_CPUS.
     usable_cpus = os.cpu_count()
-    return min(usable_cpus, config_nr_cpus)
+    return min(usable_cpus, int(config_nr_cpus))
 
 
 def setup_cfg(args):

utils.py

@@ -30,7 +30,7 @@ def die(string):
     sys.exit(1)
 
 
-def find_first_file(relative_root, possible_files):
+def find_first_file(relative_root, possible_files, required=True):
     """
     Attempts to find the first option available in the list of files relative
     to a specified root folder.
@@ -41,6 +41,8 @@ def find_first_file(relative_root, possible_files):
         possible_files (list): A list of Paths that may be within the relative
                                root folder. They will be automatically appended
                                to relative_root.
+        required (bool): Whether or not the requested file is required for the
+                         script to work properly.
     Returns:
         The full path to the first file found in the list. If none could be
         found, an Exception is raised.
@@ -49,10 +51,13 @@ def find_first_file(relative_root, possible_files):
         if (full_path := relative_root.joinpath(possible_file)).exists():
             return full_path
 
-    files_str = "', '".join([str(elem) for elem in possible_files])
-    raise Exception(
-        f"No files from list ('{files_str}') could be found within '{relative_root}'!"
-    )
+    if required:
+        files_str = "', '".join([str(elem) for elem in possible_files])
+        raise Exception(
+            f"No files from list ('{files_str}') could be found within '{relative_root}'!"
+        )
+
+    return None
 
 
 def get_full_kernel_path(kernel_location, image, arch=None):