MachineOptions snapshot_mode to control snapshot_file creation

lib/tinykvm/common.hpp

@@ -97,9 +97,20 @@ namespace tinykvm
 		/* Enable file-backed memory mappings for large files */
 		bool mmap_backed_files = false;
 		/* Enable VM snapshot by file-mapping all physical memory
-		   to the given file. The file is created if it does not exist,
+		   to the given file. Depending on `snapshot_mode`,
+		   the file may be created if it does not exist,
 		   and must be of the correct size if it does exist. */
 		std::string snapshot_file;
+		enum SnapshotMode {
+			Disabled = 0,
+			Open = 1,
+			Create = 2,
+			OpenOrCreate = 3,
+		};
+		/* When using a snapshot_file, control whether file
+		   should be created if missing, opened, or created
+		   and possibly overwritten. */
+		SnapshotMode snapshot_mode = OpenOrCreate;
 		/* When using hugepages, cover the given size with
 		   hugepages, unless 0, in which case the entire
 		   main memory will be covered. */

lib/tinykvm/memory.cpp

@@ -397,12 +397,18 @@ vMemory::AllocationResult
 	if (filename.empty()) {
 		throw std::runtime_error("No VM snapshot file specified");
 	}
-	int fd = open(filename.c_str(), O_RDONLY | O_CLOEXEC);
-	if (fd < 0) {
-		if (errno != ENOENT) {
+	if (options.snapshot_mode == MachineOptions::SnapshotMode::Disabled) {
+		throw std::runtime_error("VM snapshot disabled");
+	}
+	int fd = -1;
+	if (options.snapshot_mode != MachineOptions::SnapshotMode::Create) {
+		fd = open(filename.c_str(), O_RDONLY | O_CLOEXEC);
+		if (fd < 0 && (options.snapshot_mode == MachineOptions::SnapshotMode::Open || errno != ENOENT)) {
 			throw std::runtime_error("Failed to open VM snapshot file: " + filename);
 		}
-		fd = open(filename.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+	}
+	if (fd < 0) {
+		fd = open(filename.c_str(), O_RDWR | O_CREAT | O_TRUNC | O_CLOEXEC, 0600);
 		if (fd < 0) {
 			throw std::runtime_error("Failed to create VM snapshot file: " + filename);
 		}