Messages in this thread | | | From | (Eric W. Biederman) | Date | Fri, 11 Oct 2013 03:08:43 -0700 | Subject | Re: kexec: Clearing registers just before jumping into purgatory |
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Daniel Kiper <daniel.kiper@oracle.com> writes:
> Hi, > > Could you explain why do you clear all registers just before jumping > into purgatory (please look into arch/x86/kernel/relocate_kernel_64.S > for more details)? There is no any single word about that. I do not > count comment which states what is going on. purgatory on entry does > not assume any value in registers. Are you going to use that feature > for something in the future (e.g. to differentiate between callers > and/or Linux versions if it be needed)?
It has been a long time now, but as I recall the reason was to just have things well defined and to make certain that we were not accidentially exporting anything except the stack pointer for applications to depend upon.
0/NULL is a good choice because if you are expecting pointer for some strange reason interesting things happen.
purgatory is definitely not the only target and the C version of purgatory was actually written well after kexec came into existence.
Is there any particular reason why you are asking?
> By the way, interestingly it is not done if preserve_context is in > force.
Something different is done, and all of the registers should be preserved from the when the return to Linux.
In theory you can swap between to kernels with the preserve_context case. Technically I like the ability but I don't know that it has ever achieved much uptake.
Eric
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