Initial implementation of kdump for ARM

Quoted 12 lines · Per Fransson

Quoted 11 lines · Russell King - ARM Linux

Quoted 6 lines · Per Fransson

In machine_kexec an identity mapping is set up for the user space part of the virtual address space, through a call to setup_mm_for_reboot(). I believe this mapping is only necessary because the MMU is kept on and if the kexec is done to facilitate the collection of a dump it would be nice if a large part of the page table for the crashing context has not been corrupted. Don't you agree?

No. The identity mapping is there so that we can safely disable the MMU and jump to the intended address.

Quoted 6 lines

Quoted 4 lines

There is an implementation defined delay between writing the control register and the point in the instruction stream that the effect of that write is seen.

But the identity mapping is only set up for user space part.

That's because we're normally calling back into the boot loader.

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It needs to
be set up around the code which turns off the MMU for the case when there is no delay at all.

There is always a delay as the ARM architecture is pipelined. By the time the instruction which writes to the control register has got to the writeback stage, the following instruction has long since been fetched from memory, and has probably been decoded and partially executed.

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And it would still be nice to not corrupt the page table of the crashing context.

What you're then asking for is the crashing kernel to allocate 16K of memory to setup some page tables, and then context switch to that table so that the MMU can be turned off.

That's never going to be anywhere near reliable.