[Qemu-devel] [PATCH 07/46] Return path: Open a return path on QEMUFile for sockets

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Postcopy needs a method to send messages from the destination back to
the source, this is the 'return path'.

Wire it up for 'socket' QEMUFile's using a dup'd fd.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 include/migration/qemu-file.h |  8 +++++
 qemu-file.c                   | 74 +++++++++++++++++++++++++++++++++++++++----
 2 files changed, 76 insertions(+), 6 deletions(-)

diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
index df38646..ec1a342 100644
--- a/include/migration/qemu-file.h
+++ b/include/migration/qemu-file.h
@@ -87,6 +87,11 @@ typedef size_t (QEMURamSaveFunc)(QEMUFile *f, void *opaque,
 #define QEMUFILE_IO_BUF_SIZE 32768
 #define QEMUFILE_MAX_IOV_SIZE MIN(IOV_MAX, 64)
 
+/*
+ * Return a QEMUFile for comms in the opposite direction
+ */
+typedef QEMUFile *(QEMURetPathFunc)(void *opaque);
+
 typedef struct QEMUFileOps {
     QEMUFilePutBufferFunc *put_buffer;
     QEMUFileGetBufferFunc *get_buffer;
@@ -97,6 +102,7 @@ typedef struct QEMUFileOps {
     QEMURamHookFunc *after_ram_iterate;
     QEMURamHookFunc *hook_ram_load;
     QEMURamSaveFunc *save_page;
+    QEMURetPathFunc *get_return_path;
 } QEMUFileOps;
 
 struct QEMUFile {
@@ -117,6 +123,7 @@ struct QEMUFile {
 
     int last_error;
 
+    struct QEMUFile *return_path;
     MigrationIncomingState *mis;
 };
 
@@ -202,6 +209,7 @@ void qemu_file_set_rate_limit(QEMUFile *f, int64_t new_rate);
 int64_t qemu_file_get_rate_limit(QEMUFile *f);
 int qemu_file_get_error(QEMUFile *f);
 void qemu_file_set_error(QEMUFile *f, int ret);
+QEMUFile *qemu_file_get_return_path(QEMUFile *f);
 void qemu_fflush(QEMUFile *f);
 
 static inline void qemu_put_be64s(QEMUFile *f, const uint64_t *pv)
diff --git a/qemu-file.c b/qemu-file.c
index 88cacc7..98a6d2a 100644
--- a/qemu-file.c
+++ b/qemu-file.c
@@ -16,6 +16,54 @@ typedef struct QEMUFileSocket {
     QEMUFile *file;
 } QEMUFileSocket;
 
+/* Give a QEMUFile* off the same socket but data in the opposite
+ * direction.
+ * qemu_fopen_socket marks write fd's as blocking, but doesn't
+ * touch read fd's status, so we dup the fd just to keep settings
+ * separate. [TBD: Do I need to explicitly mark as non-block on read?]
+ */
+static QEMUFile *socket_dup_return_path(void *opaque)
+{
+    QEMUFileSocket *qfs = opaque;
+    int revfd;
+    bool this_is_read;
+    QEMUFile *result;
+
+    /* If it's already open, return it */
+    if (qfs->file->return_path) {
+        return qfs->file->return_path;
+    }
+
+    if (qemu_file_get_error(qfs->file)) {
+        /* If the forward file is in error, don't try and open a return */
+        return NULL;
+    }
+
+    /* I don't think there's a better way to tell which direction 'this' is */
+    this_is_read = qfs->file->ops->get_buffer != NULL;
+
+    revfd = dup(qfs->fd);
+    if (revfd == -1) {
+        error_report("Error duplicating fd for return path: %s",
+                      strerror(errno));
+        return NULL;
+    }
+
+    qemu_set_nonblock(revfd);
+    result = qemu_fopen_socket(revfd, this_is_read ? "wb" : "rb");
+    qfs->file->return_path = result;
+
+    if (result) {
+        /* We are the reverse path of our reverse path (although I don't
+           expect this to be used, it would stop another dup if it was */
+        result->return_path = qfs->file;
+    } else {
+        close(revfd);
+    }
+
+    return result;
+}
+
 static ssize_t socket_writev_buffer(void *opaque, struct iovec *iov, int iovcnt,
                                     int64_t pos)
 {
@@ -313,17 +361,31 @@ QEMUFile *qemu_fdopen(int fd, const char *mode)
 }
 
 static const QEMUFileOps socket_read_ops = {
-    .get_fd =     socket_get_fd,
-    .get_buffer = socket_get_buffer,
-    .close =      socket_close
+    .get_fd          = socket_get_fd,
+    .get_buffer      = socket_get_buffer,
+    .close           = socket_close,
+    .get_return_path = socket_dup_return_path
 };
 
 static const QEMUFileOps socket_write_ops = {
-    .get_fd =     socket_get_fd,
-    .writev_buffer = socket_writev_buffer,
-    .close =      socket_close
+    .get_fd          = socket_get_fd,
+    .writev_buffer   = socket_writev_buffer,
+    .close           = socket_close,
+    .get_return_path = socket_dup_return_path
 };
 
+/*
+ * Result: QEMUFile* for a 'return path' for comms in the opposite direction
+ *         NULL if not available
+ */
+QEMUFile *qemu_file_get_return_path(QEMUFile *f)
+{
+    if (!f->ops->get_return_path) {
+        return NULL;
+    }
+    return f->ops->get_return_path(f->opaque);
+}
+
 bool qemu_file_mode_is_not_valid(const char *mode)
 {
     if (mode == NULL ||
-- 
1.9.3