Re: [PATCH 00/03] kexec: Avoid overwriting the current pgd (V2)

[Magnus Damm <magnus@valinux.co.jp>]

On Wed, 2006-05-24 at 18:56 -0400, Vivek Goyal wrote:
> On Wed, May 24, 2006 at 01:40:31PM +0900, Magnus Damm wrote:
> > kexec: Avoid overwriting the current pgd (V2)
> > 
> > This patch updates the kexec code for i386 and x86_64 to avoid overwriting
> > the current pgd. For most people is overwriting the current pgd is not a big
> > problem. When kexec:ing into a new kernel that reinitializes and makes use of 
> > all memory we don't care about saving state.
> > 
> > But overwriting the current pgd is not a good solution in the case of kdump 
> > (CONFIG_CRASH_DUMP) where we want to preserve as much state as possible when 
> > a crash occurs. This patch solves the overwriting issue.
> > 
> > 20060524: V2
> > 
> > - Broke out architecture-specific data structures into asm/kexec.h
> > - Fixed a i386/PAE page table problem only triggering on real hardware.
> > - Moved segment handling code into the assembly routines.
> 
> What's the advantage of moving segment handling code into assembly
> routines? It will only add to the fear of control code page size growing
> beyond 4K.

I have two main reasons:

- Why wrap assembler instructions in C code if you just can move them
into an already existing assembly file? Much cleaner IMO.

- I'm currently working on making kexec to work under xen/dom0. And by
moving the segment handling code into the assembly file we reduce the
amount of duplicated code.

I was concerned about the size of the assembly code too and I just
re-checked with my patches applied:

i386: 300 / 4096 bytes used
x86_64: 364 / 4096 bytes used

I'd say we have more than enough space!

Thanks,

/ magnus


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