guix system: Add '--share' and '--expose' options for 'vm'.

* guix/scripts/system.scm (system-derivation-for-action): Add #:mappings
  parameter.  Pass it to 'system-qemu-image/shared-store-script'.
  (perform-action): Likewise.
  (show-help): Document --share and --expose.
  (specification->file-system-mapping): New procedure.
  (%options): Add --share and --expose.
  (guix-system): Pass #:mapping to 'perform-action'.
* doc/guix.texi (Invoking guix system): Document it.

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 730b6a3770..569790065f 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -4375,12 +4375,27 @@ This command also installs GRUB on the device specified in
 
 @item vm
 @cindex virtual machine
+@cindex VM
 Build a virtual machine that contain the operating system declared in
 @var{file}, and return a script to run that virtual machine (VM).
 Arguments given to the script are passed as is to QEMU.
 
 The VM shares its store with the host system.
 
+Additional file systems can be shared between the host and the VM using
+the @code{--share} and @code{--expose} command-line options: the former
+specifies a directory to be shared with write access, while the latter
+provides read-only access to the shared directory.
+
+The example below creates a VM in which the user's home directory is
+accessible read-only, and where the @file{/exchange} directory is a
+read-write mapping of the host's @file{$HOME/tmp}:
+
+@example
+guix system vm my-config.scm \
+   --expose=$HOME --share=$HOME/tmp=/exchange
+@end example
+
 On GNU/Linux, the default is to boot directly to the kernel; this has
 the advantage of requiring only a very tiny root disk image since the
 host's store can then be mounted.