Re: [v2 0/5] arm64: add kdump support

[AKASHI Takahiro]

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:
> > 1) "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.
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 was added.


> > 2) Kvm will not be enabled on crash dump kernel even if configured
> > See commit messages and Documentation/kdump/kdump.txt for details.
> >
> > The only concern I have is whether or not we can use the exact same kernel
> > as both system kernel and crash dump kernel. The current arm64 kernel is
> > not relocatable in the exact sense but I see no problems in using the same
> > binary when testing kdump.
>
> Ard has been working on decoupling the kernel text/data, FDT, and linear
> memory mappings, which would allow the kernel to be loaded anywhere and
> still be able to access all of memory [3]. I assume that's what you mean
> by "relocatable"?

I'm still trying to understand Ard's patchset, but I think yes.

> > I tested the code with
> > 	- ATF v1.1 + EDK2(UEFI) v3.0-rc0
> > 	- kernel v4.0 + Geoff' kexec v9
> > on
> > 	- Base fast model, and
> > 	- MediaTek MT8173-EVB
> > using my own kexec-tools [1], currently v0.12.
> >
> > You may want to start a kernel with the following boot parameter:
> > 	crashkernel=64M (or so, on model)
> > and try
> > 	$ kexec -p --load <vmlinux> --append ...
> > 	$ echo c > /proc/sysrq-trigger
> >
> > To examine vmcore (/proc/vmcore), you should use
> > 	- gdb v7.7 or later
> > 	- crash + a small patch (to recognize v4.0 kernel)
> >
> > Changes from v1:
> > * rebased to Geoff's v9
> > * tested this patchset on real hardware and fixed bugs:
> > - added cache flush operation in ipi_cpu_stop() when shutting down
> >    the system. Otherwise, data saved in vmcore's note sections by
> >    crash_save_cpu() might not be flushed to dumped memory and crash command
> >    fail to fetch correct data.
>
> We'll need to give this a go on something with a system cache too (e.g.
> Seattle or X-Gene). Even if that's only UP it would give me much greater
> confidence in the cache maintenance.

Is there any genric interface to do so?

> >    I will address Mark's commit[2] after Geoff takes care of it on kexec.
> > - modified to use ioremap_cache() instead of ioremap() when reading
> >    crash memory. Otherwise, accessing /proc/vmcore on crash dump kernel
> >    might cause an alignment fault.
> > * allows reserve_crashkernel() to handle "crashkernel=xyz[MG]" correctly,
> >    thanks to Pratyush Anand. And it now also enforces memory limit.
>
> I worry that there could be potentially bad interaction between this and
> Ard's patches, depending on how the memory area to use is chosen. It is
> probably fine, but we should make sure that it is.

I'm not sure what the point is, but I will try to check it.

Thanks,
-Takahiro AKASHI

> > * moved reserve_crashkernel() and reserve_elfcorehdr() to
> >    arm64_memblock_init() to clarify that they should be called before
> >    dma_contignuous_reserve().
> >
> > [1] https://git.linaro.org/people/takahiro.akashi/kexec-tools.git
> > [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-April/338171.html
>
> [3] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-April/337596.html
>
> Thanks,
> Mark.
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