x86, kdump: Retore crashkernel= to allocate under 896M

Vivek found old kexec-tools does not work new kernel anymore.

So change back crashkernel= back to old behavoir, and add crashkernel_high=
to let user decide if buffer could be above 4G, and also new kexec-tools will
be needed.

-v2: let crashkernel=X override crashkernel_high=
    update description about _high will be ignored by crashkernel=X
-v3: update description about kernel-parameters.txt according to Vivek.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1366089828-19692-4-git-send-email-yinghai@kernel.org
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 11 insertions, 2 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index cff672da2486..709eb3edc6b2 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -603,9 +603,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         a memory unit (amount[KMG]). See also
                         Documentation/kdump/kdump.txt for an example.
 
+        crashkernel_high=size[KMG]
+                        [KNL, x86_64] range could be above 4G. Allow kernel
+                        to allocate physical memory region from top, so could
+                        be above 4G if system have more than 4G ram installed.
+                        Otherwise memory region will be allocated below 4G, if
+                        available.
+                        It will be ignored if crashkernel=X is specified.
         crashkernel_low=size[KMG]
-                        [KNL, x86_64] range under 4G. When crashkernel= is
-                        passed, kernel allocate physical memory region
+                        [KNL, x86_64] range under 4G. When crashkernel_high= is
+                        passed, kernel could allocate physical memory region
                         above 4G, that cause second kernel crash on system
                         that require some amount of low memory, e.g. swiotlb
                         requires at least 64M+32K low memory.  Kernel would
@@ -613,6 +620,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         This one let user to specify own low range under 4G
                         for second kernel instead.
                         0: to disable low allocation.
+                        It will be ignored when crashkernel_high=X is not used
+                        or memory reserved is below 4G.
 
         cs89x0_dma=     [HW,NET]
                         Format: <dma>