diff --git a/appvm/qubes_add_pendrive_script.c b/appvm/qubes_add_pendrive_script.c
index d8e4ad26..d931ba9d 100644
--- a/appvm/qubes_add_pendrive_script.c
+++ b/appvm/qubes_add_pendrive_script.c
@@ -143,6 +143,8 @@ void suicide(struct xs_handle *xs)
 	exit(1);
 }
 
+// we are DVM, AppVM sends us a document to work on
+// /dev/xvdh contains the dvm_header and document data 
 void dvm_transaction_request(char *seq, struct xs_handle *xs)
 {
 	char filename[1024], cmdbuf[1024];
@@ -189,12 +191,16 @@ void dvm_transaction_request(char *seq, struct xs_handle *xs)
 	if (stat_pre.st_mtime == stat_post.st_mtime)
 		suicide(xs);
 	xs_daemon_close(xs);
+	// if the modify timestamp of the document file has changed, write back the
+	// modified content to the requestor AppVM
 	execl("/usr/lib/qubes/qvm-dvm-transfer", "qvm-dvm-transfer", src_vm,
 	      filename, seq, NULL);
 	syslog(LOG_DAEMON | LOG_ERR, "execl qvm-dvm-transfer");
 	suicide(xs);
 }
 
+// we are AppVM, DVM sends us a modified document
+// /dev/xvdh contains the dvm_header and document data 
 void dvm_transaction_return(char *seq_string, struct xs_handle *xs)
 {
 	int seq = strtoul(seq_string, 0, 10);
@@ -213,6 +219,7 @@ void dvm_transaction_return(char *seq_string, struct xs_handle *xs)
 		goto out_err;
 	}
 	drop_to_user();
+	// read the file name for which the open-in-dvm with transaction=="seq" was started
 	snprintf(db_name, sizeof(db_name), DBDIR "/%d", seq);
 	db_fd = open(db_name, O_RDONLY);
 	if (!db_fd) {
diff --git a/appvm/qubes_penctl.c b/appvm/qubes_penctl.c
index 878652e3..284f34cb 100644
--- a/appvm/qubes_penctl.c
+++ b/appvm/qubes_penctl.c
@@ -39,6 +39,13 @@ void check_name(unsigned char *s)
 		exit(1);
 	}
 }
+/*
+A tool to request action from qfileexchgd by writing to device/qpen xenstore key. 
+new - please attach a vfat-formatted block device at /dev/xvdg; I will either write some files to it and
+	then request sending it to other AppVM, or I will place dvm_header+some file on it and send it to DVM
+send vmname - detach my /dev/xvdg and attach it to vmname at /dev/xvdh
+umount - I am done with my /dev/xvdh, please detach it
+*/
 
 void usage(char *argv0)
 {
diff --git a/appvm/qvm-open-in-dvm.c b/appvm/qvm-open-in-dvm.c
index acfa81ce..d72989f7 100644
--- a/appvm/qvm-open-in-dvm.c
+++ b/appvm/qvm-open-in-dvm.c
@@ -62,7 +62,12 @@ int get_and_set_seq()
 	close(seq_fd);
 	return seq;
 }
-
+/*
+Write the filename we are sending to DVM to DBDIR/transaction_seq
+When DVM sends us a modified document via transaction with transaction_seq,
+we will know that we are supposed to update the document with the
+filename at DBDIR/transaction_seq
+*/ 
 void write_db(char *name, int seq)
 {
 	int db_fd;
@@ -137,6 +142,7 @@ int main(int argc, char **argv)
 		perror("xs_domain_open");
 		exit(1);
 	}
+	// request a new block device at /dev/xvdg from qfileexchgd
 	if (!xs_write(xs, 0, "device/qpen", "new", 3)) {
 		perror("xs_write");
 		exit(1);
@@ -150,8 +156,10 @@ int main(int argc, char **argv)
 	}
 	setuid(getuid());
 	if (argc == 3)
+		// we are AppVM; get new seq
 		seq = get_and_set_seq();
 	else
+		// we are DVM; use the cmdline transaction_seq
 		seq = strtoul(argv[3], 0, 0);
 	file_fd = open(abs_filename, O_RDONLY);
 	if (file_fd < 0) {
@@ -165,6 +173,9 @@ int main(int argc, char **argv)
 	copy_file(xvdg_fd, file_fd);
 	close(file_fd);
 	close(xvdg_fd);
+	// request qfileexchgd to send our /dev/xvdg to its destination
+	// either "disposable", which means "create DVM for me"
+	// or vmname, meaning this is a reply to originator AppVM
 	snprintf(buf, sizeof(buf), "send %s %d", argv[1], seq);
 	if (!xs_write(xs, 0, "device/qpen", buf, strlen(buf))) {
 		perror("xs_write");
diff --git a/dom0/pendrive_swapper/README b/dom0/pendrive_swapper/README
new file mode 100644
index 00000000..c05f863c
--- /dev/null
+++ b/dom0/pendrive_swapper/README
@@ -0,0 +1,62 @@
+	qfilexchgd is a daemon responsible for managing exchange of block
+devices ("virtual pendrives") between VMs. It is used for
+a) copying files between AppVMs
+b) copying a single file between an AppVM and a DVM
+
+	qfilexchgd is event driven. The sources of events are:
+a) trigger of xenstore watch for the changes in /local/domain xenstore hierarchy - 
+to detect start/stop of VMs, and maintain vmname->vm_xid dictionary
+b) triger of xenstore watch for a change in /local/domain/domid/device/qpen
+key - VMs write to this key to request service from qfilexchgd
+	
+	Copying files between AppVMs is implemented as follows:
+1) AppVM1 user runs qvm-copy-to-vm script (accessible from Dolphin file manager by
+"right click on a file(s)->Actions->Send to VM" menu). It calls
+"/usr/lib/qubes/qubes_penctl new", and it writes "new" request to its device/qpen 
+key. qfilexchgd creates a new 1G file, makes vfat fs on it, and does block-attach 
+so that this file is attached as /dev/xvdg in AppVM1.
+2) AppVM1 mounts /dev/xvdg on /mnt/outgoing and copies some files there,
+then unmounts it.
+3) AppVM1 writes "send DestVM" request to its device/qpen key (calling
+"/usr/lib/qubes/qubes_penctl send DestVM"). After getting confirmation by
+displaying a dialog box in dom0 display, qfilexchgd detaches /dev/xvdg
+from AppVM1, attaches it as /dev/xvdh to DestVM.
+4) In DestVM, udev script for /dev/xvdh named qubes_add_pendrive_script (see 
+/etc/udev/rules.d/qubes.rules) mounts /dev/xvdh on /mnt/incoming, and then
+waits for /mnt/incoming to become unmounted. A file manager
+running in DestVM shows a new volume, and user in DestVM may copy files from
+it. When user in DestVM is done, then user unmounts /dev/xvdh.
+qubes_add_pendrive_script then tells qfilexchgd to detach /dev/xvdh and
+terminates. 
+
+	Copying a single file between AppVM and a DVM is implemented as
+follows:
+1) User in AppVM1 runs qvm-open-in-dvm (accessible from Dolphin file manager
+by "right click on a file->Actions->Open in DVM" menu). qvm-open-in-dvm 
+a) gets a new /dev/xvdg (just as described in previous paragraph)
+b) computes a new unique transaction seq SEQ,
+c) writes the requested file name (say, /home/user/document.txt) to 
+/home/user/.dvm/SEQ.
+d) creates a dvm_header (see core.git/appvm/dvm.h) on /dev/xvdg, followed by
+file contents 
+e) writes the "send disposable SEQ" to its device/qpen xenstore key.
+2) qfilexchgd sees that "send" argument=="disposable", and creates the new
+DVM by calling /usr/lib/qubes/qubes_restore. It adds new DVM to qubesDB via
+qvm_collection.add_new_appvm. Then it attaches /dev/xvdg from AppVM1 as
+/dev/xvdh in DVM.
+3) In DVM, qubes_add_pendrive_script sees non-zero "qubes_transaction_seq"
+key in xenstore, and instead processing it as in the case of normal copy,
+treats it as a request to DVM (because we run in DVM). It retrieves the 
+body of the file passed in
+/dev/xvdh, copies to /tmp, and runs "mime-open" to open appropriate
+executable to edit it. When mime-open returns, if the file was modified,
+it is sent back to AppVM1 (by writing "send AppVM1 SEQ" to device/qpen).
+Then DVM destroys itself.
+4) In AppVM1, a new /dev/xvdh appears (because DVM sent it). 
+qubes_add_pendrive_script sees non-zero "qubes_transaction_seq" key, and 
+treats it as response from DVM (because we run in AppVM, not DVM). It 
+retrieves the filename from
+/home/user/.dvm/SEQ, and copies data from /dev/xvdh to it.
+     
+	qfilexchgd is started after first qubes_guid is started, so that it
+has access to X display in dom0 to present dialog messages.
diff --git a/dom0/restore/qubes_restore.c b/dom0/restore/qubes_restore.c
index a38aa1fe..3ba33800 100644
--- a/dom0/restore/qubes_restore.c
+++ b/dom0/restore/qubes_restore.c
@@ -197,7 +197,10 @@ void start_guid(int domid, int argc, char **argv)
 	execv("/usr/bin/qubes_guid", guid_args);
 	perror("execv");
 }
-
+// modify the savefile. fd = fd to the open savefile,
+// buf - already read 1st page of the savefile
+// pattern - pattern to search for
+// val - string to replace pattern with
 void fix_savefile(int fd, char *buf, char *pattern, char *val)
 {
 	int i, len = strlen(val), origlen;
@@ -237,6 +240,10 @@ char *build_dvm_ip(int netvm, int id)
 }
 
 #define NAME_PATTERN "/root-cow.img"
+// replaces the unique portions of the savefile with per-dvm values
+// returns the name of VM the savefile was taken for 
+// by looking for /.../vmname/root-cow.img
+// normally, it should be "templatename-dvm"
 char *get_vmname_from_savefile(int fd)
 {
 	static char buf[4096];