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

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
Cc: <stable@vger.kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 6c723811c0a0..c4cc199e32cb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -754,6 +754,7 @@ 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#]
@@ -771,6 +772,8 @@ 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