Since the commit 25b5ff1a86 ("qga: add mountpoint usage info to
GuestFilesystemInfo") we have 2 values reported in guest-get-fsinfo:
used = (f_blocks - f_bfree), total = (f_blocks - f_bfree + f_bavail) as
returned by statvfs(3).  While on Windows guests that's all we can get
with GetDiskFreeSpaceExA(), on POSIX guests we might also be interested in
total file system size, as it's visible for root user.  Let's add an
optional field 'total-bytes-root' to GuestFilesystemInfo struct, which'd
only be reported on POSIX and represent f_blocks value as returned by
statvfs(3).

While here, let's document better where those values come from in both
POSIX and Windows.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 qga/commands-posix.c |  2 ++
 qga/commands-win32.c |  1 +
 qga/qapi-schema.json | 12 +++++++++++-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 26008db497..8207c4c47e 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -1569,8 +1569,10 @@ static GuestFilesystemInfo *build_guest_fsinfo(struct FsMount *mount,
         nonroot_total = used + buf.f_bavail;
         fs->used_bytes = used * fr_size;
         fs->total_bytes = nonroot_total * fr_size;
+        fs->total_bytes_root = buf.f_blocks * fr_size;
 
         fs->has_total_bytes = true;
+        fs->has_total_bytes_root = true;
         fs->has_used_bytes = true;
     }
 
diff --git a/qga/commands-win32.c b/qga/commands-win32.c
index a1015757d8..9e820aad8d 100644
--- a/qga/commands-win32.c
+++ b/qga/commands-win32.c
@@ -1143,6 +1143,7 @@ static GuestFilesystemInfo *build_guest_fsinfo(char *guid, Error **errp)
     fs = g_malloc(sizeof(*fs));
     fs->name = g_strdup(guid);
     fs->has_total_bytes = false;
+    fs->has_total_bytes_root = false;
     fs->has_used_bytes = false;
     if (len == 0) {
         fs->mountpoint = g_strdup("System Reserved");
diff --git a/qga/qapi-schema.json b/qga/qapi-schema.json
index b8efe31897..093a5ab602 100644
--- a/qga/qapi-schema.json
+++ b/qga/qapi-schema.json
@@ -1031,8 +1031,18 @@
 # @type: file system type string
 #
 # @used-bytes: file system used bytes (since 3.0)
+#     * POSIX: (f_blocks - f_bfree) * f_frsize, as returned by statvfs(3)
+#     * Windows: (TotalNumberOfBytes - TotalNumberOfFreeBytes), as returned
+#       by GetDiskFreeSpaceEx()
 #
 # @total-bytes: non-root file system total bytes (since 3.0)
+#     * POSIX: (f_blocks - f_bfree + f_bavail) * f_frsize, as returned by
+#       statvfs(3)
+#     * Windows: TotalNumberOfBytes, as returned by GetDiskFreeSpaceEx()
+#
+# @total-bytes-root: total file system size in bytes (as visible for a
+#     priviledged user) (since 8.3)
+#     * POSIX only: (f_blocks * f_frsize), returned by statvfs(3)
 #
 # @disk: an array of disk hardware information that the volume lies
 #     on, which may be empty if the disk type is not supported
@@ -1042,7 +1052,7 @@
 { 'struct': 'GuestFilesystemInfo',
   'data': {'name': 'str', 'mountpoint': 'str', 'type': 'str',
            '*used-bytes': 'uint64', '*total-bytes': 'uint64',
-           'disk': ['GuestDiskAddress']} }
+           '*total-bytes-root': 'uint64', 'disk': ['GuestDiskAddress']} }
 
 ##
 # @guest-get-fsinfo:
-- 
2.39.3