migration/qemu-file: add utility methods for working with seekable channels

Add utility methods that will be needed when implementing 'mapped-ram'
migration capability.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Link: https://lore.kernel.org/r/20240229153017.2221-7-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

include/migration/qemu-file-types.h

@@ -50,6 +50,8 @@ unsigned int qemu_get_be16(QEMUFile *f);
 unsigned int qemu_get_be32(QEMUFile *f);
 uint64_t qemu_get_be64(QEMUFile *f);
 
+bool qemu_file_is_seekable(QEMUFile *f);
+
 static inline void qemu_put_be64s(QEMUFile *f, const uint64_t *pv)
 {
     qemu_put_be64(f, *pv);

migration/qemu-file.c

@@ -33,6 +33,7 @@
 #include "options.h"
 #include "qapi/error.h"
 #include "rdma.h"
+#include "io/channel-file.h"
 
 #define IO_BUF_SIZE 32768
 #define MAX_IOV_SIZE MIN_CONST(IOV_MAX, 64)
@@ -255,6 +256,10 @@ static void qemu_iovec_release_ram(QEMUFile *f)
     memset(f->may_free, 0, sizeof(f->may_free));
 }
 
+bool qemu_file_is_seekable(QEMUFile *f)
+{
+    return qio_channel_has_feature(f->ioc, QIO_CHANNEL_FEATURE_SEEKABLE);
+}
 
 /**
  * Flushes QEMUFile buffer
@@ -447,6 +452,107 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
     }
 }
 
+void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
+                        off_t pos)
+{
+    Error *err = NULL;
+    size_t ret;
+
+    if (f->last_error) {
+        return;
+    }
+
+    qemu_fflush(f);
+    ret = qio_channel_pwrite(f->ioc, (char *)buf, buflen, pos, &err);
+
+    if (err) {
+        qemu_file_set_error_obj(f, -EIO, err);
+        return;
+    }
+
+    if ((ssize_t)ret == QIO_CHANNEL_ERR_BLOCK) {
+        qemu_file_set_error_obj(f, -EAGAIN, NULL);
+        return;
+    }
+
+    if (ret != buflen) {
+        error_setg(&err, "Partial write of size %zu, expected %zu", ret,
+                   buflen);
+        qemu_file_set_error_obj(f, -EIO, err);
+        return;
+    }
+
+    stat64_add(&mig_stats.qemu_file_transferred, buflen);
+
+    return;
+}
+
+
+size_t qemu_get_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
+                          off_t pos)
+{
+    Error *err = NULL;
+    size_t ret;
+
+    if (f->last_error) {
+        return 0;
+    }
+
+    ret = qio_channel_pread(f->ioc, (char *)buf, buflen, pos, &err);
+
+    if ((ssize_t)ret == -1 || err) {
+        qemu_file_set_error_obj(f, -EIO, err);
+        return 0;
+    }
+
+    if ((ssize_t)ret == QIO_CHANNEL_ERR_BLOCK) {
+        qemu_file_set_error_obj(f, -EAGAIN, NULL);
+        return 0;
+    }
+
+    if (ret != buflen) {
+        error_setg(&err, "Partial read of size %zu, expected %zu", ret, buflen);
+        qemu_file_set_error_obj(f, -EIO, err);
+        return 0;
+    }
+
+    return ret;
+}
+
+void qemu_set_offset(QEMUFile *f, off_t off, int whence)
+{
+    Error *err = NULL;
+    off_t ret;
+
+    if (qemu_file_is_writable(f)) {
+        qemu_fflush(f);
+    } else {
+        /* Drop all cached buffers if existed; will trigger a re-fill later */
+        f->buf_index = 0;
+        f->buf_size = 0;
+    }
+
+    ret = qio_channel_io_seek(f->ioc, off, whence, &err);
+    if (ret == (off_t)-1) {
+        qemu_file_set_error_obj(f, -EIO, err);
+    }
+}
+
+off_t qemu_get_offset(QEMUFile *f)
+{
+    Error *err = NULL;
+    off_t ret;
+
+    qemu_fflush(f);
+
+    ret = qio_channel_io_seek(f->ioc, 0, SEEK_CUR, &err);
+    if (ret == (off_t)-1) {
+        qemu_file_set_error_obj(f, -EIO, err);
+    }
+    return ret;
+}
+
+
 void qemu_put_byte(QEMUFile *f, int v)
 {
     if (f->last_error) {

migration/qemu-file.h

@@ -75,6 +75,12 @@ QEMUFile *qemu_file_get_return_path(QEMUFile *f);
 int qemu_fflush(QEMUFile *f);
 void qemu_file_set_blocking(QEMUFile *f, bool block);
 int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size);
+void qemu_set_offset(QEMUFile *f, off_t off, int whence);
+off_t qemu_get_offset(QEMUFile *f);
+void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
+                        off_t pos);
+size_t qemu_get_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
+                          off_t pos);
 
 QIOChannel *qemu_file_get_ioc(QEMUFile *file);