Re: [v2 4/5] arm64: add kdump support

From: AKASHI Takahiro
Date: Mon May 11 2015 - 03:59:06 EST


On 05/11/2015 04:47 PM, Dave Young wrote:
On 05/08/15 at 08:19pm, Dave Young wrote:

On 04/24/15 at 04:53pm, AKASHI Takahiro wrote:
Please read the following commits for arm64-specific constraints:
arm64: kdump: reserve memory for crash dump kernel
arm64: kdump: do not go into EL2 before starting a crash dump kernel

Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
Documentation/kdump/kdump.txt | 31 ++++++++++++++++++++++++++++++-
arch/arm64/Kconfig | 12 ++++++++++++
2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index bc4bd5a..f9cf6f5 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 +249,29 @@ Dump-capture kernel config options (Arch Dependent, arm)


+Dump-capture kernel config options (Arch Dependent, arm64)
+1) Disable symmetric multi-processing support under "Processor type and
+ features":
+ (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
+ when loading the dump-capture kernel, see section "Load the Dump-capture
+ Kernel".)
+2) Under uefi, 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.

I noticed you are passing mem=X in kexec-tools, Pratyush found a problem
that vmcore is corrupted because kdump kernel is using the crash notes memory.

How does kdump kernel know the system ram range especially the mem_start?
Only with the size from mem=? parammeter?

In X86 because it use E820 so kexec-tools can pass the memory range to kdump
kernel even for UEFI booting. But in arm64 may need find other way to
communicate with 2nd kernel like memmap=exactmap in X86..

Discussed with Pratyush in irc, he mentioned that start address is get from
kernel->entry which was passed by kexec-tools.

So sorry for the noise, it will works fine with the UEFI mem= limitation
fix from Mark Salter

No, not at all.
As you know, in another thread (with Mark), we're discussing this.
If we conclude that we should go for "memmap=" approach, I will update my patch.

-Takahiro AKASHI

+3) Currently, kvm will not be initialized on the dump-capture kernel even
+ if it is configured.
Extended crashkernel syntax

@@ -312,6 +335,7 @@ Boot into System Kernel
any space below the alignment point may be overwritten by the dump-capture kernel,
which means it is possible that the vmcore is not that precise as expected.

+ On arm64, use "crashkernel=Y@X".

Load the Dump-capture Kernel
@@ -334,6 +358,8 @@ For s390x:
- Use image or bzImage
For arm:
- Use zImage
+For arm64:
+ - Use vmlinux

If you are using a uncompressed vmlinux image then use following command
to load dump-capture kernel.
@@ -377,6 +403,9 @@ For s390x:
For arm:
"1 maxcpus=1 reset_devices"

+For arm64:
+ "1 maxcpus=1 mem=X[MG] reset_devices"
Notes on loading the dump-capture kernel:

* By default, the ELF headers are stored in ELF64 format to support
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5716edf..8e2f545 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -562,6 +562,18 @@ config KEXEC
but it is independent of the system firmware. And like a reboot
you can start any kernel with it, not just Linux.

+config CRASH_DUMP
+ bool "Build kdump crash kernel"
+ help
+ Generate crash dump after being started by kexec. This should
+ be normally only set in special crash dump kernels which are
+ loaded in the main kernel with kexec-tools into a specially
+ reserved region and then later executed after a crash by
+ kdump/kexec. The crash dump kernel must be compiled to a
+ memory address not used by the main kernel.
+ For more details see Documentation/kdump/kdump.txt
config XEN_DOM0
def_bool y
depends on XEN

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