Re: [v2 0/5] arm64: allow to reserve memory for normal kexec kernel

[Dave Young]

On 07/09/19 at 02:20pm, Pavel Tatashin wrote:
> Changelog
> v1 - v2
> 	- No changes to patches, addressed suggestion from James Morse
> 	  to add "arm64" tag to cover letter.
> 	- Improved cover letter information based on discussion.
> 
> Currently, it is only allowed to reserve memory for crash kernel, because
> it is a requirement in order to be able to boot into crash kernel without
> touching memory of crashed kernel is to have memory reserved.
> 
> The second benefit for having memory reserved for kexec kernel is
> that it does not require a relocation after segments are loaded into
> memory.
> 
> If kexec functionality is used for a fast system update, with a minimal
> downtime, the relocation of kernel + initramfs might take a significant
> portion of reboot.
> 
> In fact, on the machine that we are using, that has ARM64 processor
> it takes 0.35s to relocate during kexec, thus taking 52% of kernel reboot
> time:
> 
> kernel shutdown	0.03s
> relocation	0.35s
> kernel startup	0.29s
> 
> Image: 13M and initramfs is 24M. If initramfs increases, the relocation
> time increases proportionally.
> 
> While, it is possible to add 'kexeckernel=' parameters support to other
> architectures by modifying reserve_crashkernel(), in this series this is
> done for arm64 only.
> 
> The reason it is so slow on arm64 to relocate kernel is because the code
> that does relocation does this with MMU disabled, and thus D-Cache and
> I-Cache must also be disabled.
> 
> Alternative solution is more complicated: Setup a temporary page table
> for relocation_routine and also for code from cpu_soft_restart. Perform
> relocation with MMU enabled, do cpu_soft_restart where MMU and caching
> are disabled, jump to purgatory. A similar approach was suggested for
> purgatory and was rejected due to making purgatory too complicated.

The crashkernel reservation for kdump is a must, there are already a lot
of different problems need to consider, for example the low and high
memory issues, and a lot of other things.  I'm not convinced to enable
this for kexec reboot.

This really looks to workaround the arm64 issue and move the
complication to kernel.

> On, the other hand hibernate does something similar already, but there
> MMU never needs to be disabled, and also by the time machine_kexec()
> is called, allocator is not available, as we can't fail to do reboot,
> so page table must be pre-allocated during kernel load time.
> 
> Note: the above time is relocation time only. Purgatory usually also
> computes checksum, but that is skipped, because --no-check is used when
> kernel image is loaded via kexec.
> 
> Pavel Tatashin (5):
>   kexec: quiet down kexec reboot
>   kexec: add resource for normal kexec region
>   kexec: export common crashkernel/kexeckernel parser
>   kexec: use reserved memory for normal kexec reboot
>   arm64, kexec: reserve kexeckernel region
> 
>  .../admin-guide/kernel-parameters.txt         |  7 ++
>  arch/arm64/kernel/setup.c                     |  5 ++
>  arch/arm64/mm/init.c                          | 83 ++++++++++++-------
>  include/linux/crash_core.h                    |  6 ++
>  include/linux/ioport.h                        |  1 +
>  include/linux/kexec.h                         |  6 +-
>  kernel/crash_core.c                           | 27 +++---
>  kernel/kexec_core.c                           | 50 +++++++----
>  8 files changed, 127 insertions(+), 58 deletions(-)
> 
> -- 
> 2.22.0
> 
> 
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Thanks
Dave