doc, xen: Mention 'earlyprintk=xen' in the documentation.

commit 2482a92e7d17187301d7313cfe5021b13393a0b4 upstream.

The earlyprintk for Xen PV guests utilizes a simple hypercall
(console_io) to provide output to Xen emergency console.

Note that the Xen hypervisor should be booted with 'loglevel=all'
to output said information.

Reported-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Link: http://lkml.kernel.org/r/1361825650-14031-2-git-send-email-konrad.wilk@oracle.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index c1601e5..f2e1645 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -742,6 +742,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
        earlyprintk=    [X86,SH,BLACKFIN]
                        earlyprintk=vga
+                       earlyprintk=xen
                        earlyprintk=serial[,ttySn[,baudrate]]
                        earlyprintk=ttySn[,baudrate]
                        earlyprintk=dbgp[debugController#]
@@ -759,6 +760,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                        The VGA output is eventually overwritten by the real
                        console.
 
+                       The xen output can only be used by Xen PV guests.
+
        ekgdboc=        [X86,KGDB] Allow early kernel console debugging
                        ekgdboc=kbd