migration: file URI offset

Allow an offset option to be specified as part of the file URI, in
the form "file:filename,offset=offset", where offset accepts the common
size suffixes, or the 0x prefix, but not both.  Migration data is written
to and read from the file starting at offset.  If unspecified, it defaults
to 0.

This is needed by libvirt to store its own data at the head of the file.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <1694182931-61390-3-git-send-email-steven.sistare@oracle.com>

migration/file.c

@@ -6,6 +6,8 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/cutils.h"
+#include "qapi/error.h"
 #include "channel.h"
 #include "file.h"
 #include "migration.h"
@@ -13,14 +15,41 @@
 #include "io/channel-util.h"
 #include "trace.h"
 
-void file_start_outgoing_migration(MigrationState *s, const char *filename,
+#define OFFSET_OPTION ",offset="
+
+/* Remove the offset option from @filespec and return it in @offsetp. */
+
+static int file_parse_offset(char *filespec, uint64_t *offsetp, Error **errp)
+{
+    char *option = strstr(filespec, OFFSET_OPTION);
+    int ret;
+
+    if (option) {
+        *option = 0;
+        option += sizeof(OFFSET_OPTION) - 1;
+        ret = qemu_strtosz(option, NULL, offsetp);
+        if (ret) {
+            error_setg_errno(errp, -ret, "file URI has bad offset %s", option);
+            return -1;
+        }
+    }
+    return 0;
+}
+
+void file_start_outgoing_migration(MigrationState *s, const char *filespec,
                                    Error **errp)
 {
+    g_autofree char *filename = g_strdup(filespec);
     g_autoptr(QIOChannelFile) fioc = NULL;
+    uint64_t offset = 0;
     QIOChannel *ioc;
 
     trace_migration_file_outgoing(filename);
 
+    if (file_parse_offset(filename, &offset, errp)) {
+        return;
+    }
+
     fioc = qio_channel_file_new_path(filename, O_CREAT | O_WRONLY | O_TRUNC,
                                      0600, errp);
     if (!fioc) {
@@ -28,6 +57,9 @@ void file_start_outgoing_migration(MigrationState *s, const char *filename,
     }
 
     ioc = QIO_CHANNEL(fioc);
+    if (offset && qio_channel_io_seek(ioc, offset, SEEK_SET, errp) < 0) {
+        return;
+    }
     qio_channel_set_name(ioc, "migration-file-outgoing");
     migration_channel_connect(s, ioc, NULL, NULL);
 }
@@ -41,19 +73,28 @@ static gboolean file_accept_incoming_migration(QIOChannel *ioc,
     return G_SOURCE_REMOVE;
 }
 
-void file_start_incoming_migration(const char *filename, Error **errp)
+void file_start_incoming_migration(const char *filespec, Error **errp)
 {
+    g_autofree char *filename = g_strdup(filespec);
     QIOChannelFile *fioc = NULL;
+    uint64_t offset = 0;
     QIOChannel *ioc;
 
     trace_migration_file_incoming(filename);
 
+    if (file_parse_offset(filename, &offset, errp)) {
+        return;
+    }
+
     fioc = qio_channel_file_new_path(filename, O_RDONLY, 0, errp);
     if (!fioc) {
         return;
     }
 
     ioc = QIO_CHANNEL(fioc);
+    if (offset && qio_channel_io_seek(ioc, offset, SEEK_SET, errp) < 0) {
+        return;
+    }
     qio_channel_set_name(QIO_CHANNEL(ioc), "migration-file-incoming");
     qio_channel_add_watch_full(ioc, G_IO_IN,
                                file_accept_incoming_migration,

qemu-options.hx

@@ -4706,7 +4706,7 @@ DEF("incoming", HAS_ARG, QEMU_OPTION_incoming, \
     "                prepare for incoming migration, listen on\n" \
     "                specified protocol and socket address\n" \
     "-incoming fd:fd\n" \
-    "-incoming file:filename\n" \
+    "-incoming file:filename[,offset=offset]\n" \
     "-incoming exec:cmdline\n" \
     "                accept incoming migration on given file descriptor\n" \
     "                or from given external command\n" \
@@ -4725,8 +4725,9 @@ SRST
 ``-incoming fd:fd``
     Accept incoming migration from a given file descriptor.
 
-``-incoming file:filename``
+``-incoming file:filename[,offset=offset]``
-    Accept incoming migration from a given file.
+    Accept incoming migration from a given file starting at offset.
+    offset allows the common size suffixes, or a 0x prefix, but not both.
 
 ``-incoming exec:cmdline``
     Accept incoming migration as an output from specified external