Mark,
On 05/12/2015 02:43 PM, Dave Young wrote:
On 05/11/15 at 03:16pm, AKASHI Takahiro wrote:
Hi
Sorry for late response. I was on vacation.
On 04/24/2015 06:53 PM, Mark Rutland wrote:
Hi,
On Fri, Apr 24, 2015 at 08:53:03AM +0100, AKASHI Takahiro wrote:
This patch set enables kdump (crash dump kernel) support on arm64 on top of Geoff's kexec patchset.
In this version, there are some arm64-specific usage/constraints:
- "mem=" boot parameter must be specified on crash dump kernel if the system starts on uefi.
This sounds very painful. Why is this the case, and how do x86 and/or ia64 get around that?
As Dave (Young) said, x86 uses "memmap=XX" kernel commandline parameters to specify usable memory for crash dump kernel.
Originally x86 use memmap=exactmap memmap=XX to specify each section of memories for 2nd kernel. But later because a lot of reserved type ranges need to be passed ie. for pci mmconfig, and kernel cmdline buffer is limited so kexec-tools later switch to passing these in x86 boot params as E820 memory ranges directly.
On my arm64 implementation, "linux,usable-memory" property is added to device tree blob by kexec-tools for this purpose. This is because, when I first implemented kdump on arm64, ppc is the only architecture that supports kdump AND utilizes device trees. Since kexec-tools as well as the kernel already has this framework, I believed that device-tree approach was smarter than a commandline parameter.
However, uefi-based kernel ignores all the memory-related properties in a device tree and so this "mem=" workaround
Kdump kernel reuses the memmap info getting from firmware during 1st kernel boot, I do not think the memmap info can be cooked for crash kernel usable memory. But it might be a better way to use a special fdt node for crash kernel memory even for UEFI..
Do you still prefer "memmap=" approach?
Just FYI,
x86 arm arm64 powerpc{,64}
(a) cmdline(crashkernel=) cmdline cmdline cmdline (b) iomem iomem iomem dev tree (c) cmdline(memmap=) cmdline(mem=) dev tree dev tree & cmdline(mem=)
(a) how we specify/reserve crash dump kernel's memory on 1st kernel (b) how we inform userspace tool of crash dump kernel's memory on 1st kernel - iomem: add "Crash kernel" entry in /proc/iomem - dev tree: add /chosen/linux,crashkernel-base (and crashkernel-size) (c) how we specify usable memory on crash dump kernel - dev tree: add /chosen/memory/linux,usable-memory
As you see, arm64, and arm as well, does things in a *hybrid* way of x86 and ppc. I think we should go for a uniform way with device tree as ppc does. (Even for (a), we can use a device tree.)
Another way is introducing a similar memmap=, but maybe consider only system_ram type ranges. For other memory areas still use UEFI memmap.
Yeah, I don't know whether we need additional formats of memmap=, like memmap=XX$YY or memmap=XX#YY, for ACPI stuffs.
Thanks, -Takahiro AKASHI
Thanks Dave