kern/xen: Add Xen command line parsing

Xen traditionally allows customizing guest behavior by passing arguments
to the VM kernel via the kernel command line. This is no longer possible
when using GRUB with Xen, as the kernel command line is decided by the
GRUB configuration file within the guest, not data passed to the guest
by Xen.

To work around this limitation, enable GRUB to parse a command line
passed to it by Xen, and expose data from the command line to the GRUB
configuration as environment variables. These variables can be used in
the GRUB configuration for any desired purpose, such as extending the
kernel command line passed to the guest. The command line format is
inspired by the Linux kernel's command line format.

To reduce the risk of misuse, abuse, or accidents in production, the
command line will only be parsed if it consists entirely of 7-bit ASCII
characters, only alphabetical characters and underscores are permitted
in variable names, and all variable names must start with the string
"xen_grub_env_". This also allows room for expanding the command line
arguments accepted by GRUB in the future, should other arguments end up
becoming desirable in the future.

Signed-off-by: Aaron Rainbolt <arraybolt3@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -3271,6 +3271,7 @@ GRUB.  Others may be used freely in GRUB configuration files.
 @menu
 * Special environment variables::
 * Environment block::
+* Passing environment variables through Xen::
 @end menu
 
 
@@ -3871,6 +3872,57 @@ using BIOS or EFI functions (no ATA, USB or IEEE1275).
 @command{grub-mkconfig} uses this facility to implement
 @samp{GRUB_SAVEDEFAULT} (@pxref{Simple configuration}).
 
+@node Passing environment variables through Xen
+@section Passing environment variables through Xen
+
+If you are using a GRUB image as the kernel for a PV or PVH Xen virtual
+machine, you can pass environment variables from Xen's dom0 to the VM through
+the Xen-provided kernel command line. When combined with a properly configured
+guest, this can be used to customize the guest's behavior on bootup via the
+VM's Xen configuration file.
+
+GRUB will parse the kernel command line passed to it by Xen during bootup.
+The command line will be split into space-delimited words. Single and
+double quotes may be used to quote words or portions of words that contain
+spaces. Single quotes will be considered part of a word if inside double
+quotes, and vice versa. Arbitrary characters may be backslash-escaped to make
+them a literal component of a word rather than being parsed as quotes or word
+separators. The command line must consist entirely of printable 7-bit ASCII
+characters and spaces. If a non-printing ASCII character is found anywhere in
+the command line, the entire command line will be ignored by GRUB. (This
+splitter algorithm is meant to behave somewhat like Bash's word splitting.)
+
+Each word should be a variable assignment in the format ``variable'' or
+``variable=value''. Variable names must contain only the characters A-Z, a-z,
+and underscore (``_''). Variable names must begin with the string
+``xen_grub_env_''. Variable values can contain arbitrary printable 7-bit
+ASCII characters and space. If any variable contains an illegal name, that
+variable will be ignored.
+
+If a variable name and value are both specified, the variable will be set to
+the specified value. If only a variable name is specified, the variable's
+value will be set to ``1''.
+
+The following is a simple example of how to use this functionality to append
+arbitrary variables to a guest's kernel command line:
+
+@example
+# In the Xen configuration file for the guest
+name = "linux_vm"
+type = "pvh"
+kernel = "/path/to/grub-i386-xen_pvh.bin"
+extra = "xen_grub_env_linux_append='loglevel=3'"
+memory = 1024
+disk = [ "file:/srv/vms/linux_vm.img,sda,w" ]
+
+# In the guest's GRUB configuration file
+menuentry "Linux VM with dom0-specified kernel parameters" @{
+    search --set=root --label linux_vm --hint hd0,msdos1
+    linux /boot/vmlinuz root=LABEL=linux_vm $@{xen_grub_env_linux_append@}
+    initrd /boot/initrd.img
+@}
+@end example
+
 @node Modules
 @chapter Modules