[PATCH 18/19] arm64: kdump: update a kernel doc

[Dave Young]

On 01/18/16 at 07:26pm, AKASHI Takahiro wrote:
> On 01/16/2016 05:16 AM, Mark Rutland wrote:
> >On Fri, Jan 15, 2016 at 07:18:38PM +0000, Geoff Levand wrote:
> >>From: AKASHI Takahiro <takahiro.akashi at linaro.org>
> >>
> >>This patch adds arch specific descriptions about kdump usage on arm64
> >>to kdump.txt.
> >>
> >>Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
> >>---
> >>  Documentation/kdump/kdump.txt | 23 ++++++++++++++++++++++-
> >>  1 file changed, 22 insertions(+), 1 deletion(-)
> >>
> >>diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
> >>index bc4bd5a..36cf978 100644
> >>--- a/Documentation/kdump/kdump.txt
> >>+++ b/Documentation/kdump/kdump.txt
> >>@@ -18,7 +18,7 @@ memory image to a dump file on the local disk, or across the network to
> >>  a remote system.
> >>
> >>  Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
> >>-s390x and arm architectures.
> >>+s390x, arm and arm64 architectures.
> >>
> >>  When the system kernel boots, it reserves a small section of memory for
> >>  the dump-capture kernel. This ensures that ongoing Direct Memory Access
> >>@@ -249,6 +249,20 @@ Dump-capture kernel config options (Arch Dependent, arm)
> >>
> >>      AUTO_ZRELADDR=y
> >>
> >>+Dump-capture kernel config options (Arch Dependent, arm64)
> >>+----------------------------------------------------------
> >>+
> >>+1) The maximum memory size on the dump-capture kernel must be limited by
> >>+   specifying:
> >>+
> >>+   mem=X[MG]
> >>+
> >>+   where X should be less than or equal to the size in "crashkernel="
> >>+   boot parameter. Kexec-tools will automatically add this.
> >
> >
> >This is extremely fragile, and will trivially fail when the kernel can
> >be loaded anywhere (see [1]).
> 
> As I said before, this restriction also exists on arm, but I understand
> that recent Ard's patches break it.
> 
> >We must explicitly describe the set of regions the crash kernel may use
> >(i.e. we need base and size). NAK in the absence of that.
> 
> There seem to exist several approaches:
> (a) use a device-tree property, "linux,usable-memory", in addition to "reg"
>     under "memory" node
> (b) use a kernel's early parameter, "memmap=nn[@#$]ss"
> 
> Power PC takes (a), while this does not work on efi-started kernel
> because dtb has no "memory" nodes under efi.
> X86 takes (b). If we take this, we will need to overwrite a weak
> early_init_dt_add_memory().

X86 takes another way in latest kexec-tools and kexec_file_load, that is
recreating E820 table and pass it to kexec/kdump kernel, if the entries
are over E820 limitation then turn to use setup_data list for remain
entries.

I think it is X86 specific. Personally I think device tree property is
better.

Thanks
Dave