services: Add qemu-guest-agent service.

* gnu/services/virtualization.scm (<qemu-guest-agent-configuration>): New
record.
(qemu-guest-agent-shepherd-service): New procedure.
(qemu-guest-agent-service-type): New variable.
* doc/guix.texi (Virtualization Services): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -30060,6 +30060,53 @@ Return the name of @var{platform}---a string such as @code{"arm"}.
 @end deffn
 
 
+@subsubheading QEMU Guest Agent
+
+@cindex emulation
+
+The QEMU guest agent provides control over the emulated system to the
+host.  The @code{qemu-guest-agent} service runs the agent on Guix
+guests.  To control the agent from the host, open a socket by invoking
+QEMU with the following arguments:
+
+@example
+qemu-system-x86_64 \
+	-chardev socket,path=/tmp/qga.sock,server=on,wait=off,id=qga0 \
+	-device virtio-serial \
+	-device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0 \
+	...
+@end example
+
+This creates a socket at @file{/tmp/qga.sock} on the host.  Once the
+guest agent is running, you can issue commands with @code{socat}:
+
+@example
+$ guix shell socat -- socat unix-connect:/tmp/qga.sock stdio
+@{"execute": "guest-get-host-name"@}
+@{"return": @{"host-name": "guix"@}@}
+@end example
+
+See @url{https://wiki.qemu.org/Features/GuestAgent,QEMU guest agent
+documentation} for more options and commands.
+
+@defvr {Scheme Variable} qemu-guest-agent-service-type
+Service type for the QEMU guest agent service.
+@end defvr
+
+@deftp {Data Type} qemu-guest-agent-configuration
+Configuration for the @code{qemu-guest-agent} service.
+
+@table @asis
+@item @code{qemu} (default: @code{qemu-minimal})
+The QEMU package to use.
+
+@item @code{device} (default: @code{""})
+File name of the device or socket the agent uses to communicate with the
+host.  If empty, QEMU uses a default file name.
+@end table
+@end deftp
+
+
 @subsubheading The Hurd in a Virtual Machine
 
 @cindex @code{hurd}

gnu/services/virtualization.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2017 Ryan Moe <ryan.moe@gmail.com>
 ;;; Copyright © 2018, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2020,2021 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+;;; Copyright © 2021 Timotej Lazar <timotej.lazar@araneo.si>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -82,7 +83,11 @@
 
             qemu-binfmt-configuration
             qemu-binfmt-configuration?
-            qemu-binfmt-service-type))
+            qemu-binfmt-service-type
+
+            qemu-guest-agent-configuration
+            qemu-guest-agent-configuration?
+            qemu-guest-agent-service-type))
 
 (define (uglify-field-name field-name)
   (let ((str (symbol->string field-name)))
@@ -847,6 +852,47 @@ given QEMU package."
 compiled for other architectures using QEMU and the @code{binfmt_misc}
 functionality of the kernel Linux.")))
 
+
+;;;
+;;; QEMU guest agent service.
+;;;
+
+(define-configuration qemu-guest-agent-configuration
+  (qemu
+   (package qemu-minimal)
+   "QEMU package.")
+  (device
+   (string "")
+   "Path to device or socket used to communicate with the host.  If not
+specified, the QEMU default path is used."))
+
+(define qemu-guest-agent-shepherd-service
+  (match-lambda
+    (($ <qemu-guest-agent-configuration> qemu device)
+     (list
+      (shepherd-service
+       (provision '(qemu-guest-agent))
+       (documentation "Run the QEMU guest agent.")
+       (start #~(make-forkexec-constructor
+                 `(,(string-append #$qemu "/bin/qemu-ga") "--daemon"
+                   "--pidfile=/var/run/qemu-ga.pid"
+                   "--statedir=/var/run"
+                   ,@(if #$device
+                         (list (string-append "--path=" #$device))
+                         '()))
+                 #:pid-file "/var/run/qemu-ga.pid"
+                 #:log-file "/var/log/qemu-ga.log"))
+       (stop #~(make-kill-destructor)))))))
+
+(define qemu-guest-agent-service-type
+  (service-type
+   (name 'qemu-guest-agent)
+   (extensions
+    (list (service-extension shepherd-root-service-type
+                             qemu-guest-agent-shepherd-service)))
+   (default-value (qemu-guest-agent-configuration))
+   (description "Run the QEMU guest agent.")))
+
 
 ;;;
 ;;; Secrets for guest VMs.