"Huang, Ying" <ying.huang@intel.com> writes:

quoted text
> On Wed, 2008-05-14 at 14:43 -0700, Eric W. Biederman wrote:
> [...]
>> Then as a preliminary design let's plan on this.
>> 
>> - Pass the rentry point as the return address (using the C ABI).
>>   We may want to load the stack pointer etc so we can act as
>>   a direct entry point for new code.
>
> There are some issues about passing entry point as return address. The
> kexec jump (or kexec with return) is used for
>
> - Switching between original kernel (A) and kexeced kernel (B)
> - Call some code (such as BIOS code) in physical mode
>
> 1) When call some code in physical mode, the called code can use a
> simple return to return to kernel A. So there is no return address on
> stack after return to kernel A. Instead, argument 1 is on stack top.
>
> 2) When switch back from kernel B to kernel A, kernel B will call the
> jump back entry of kernel A with C ABI. So, the return address is on
> stack top. And kernel A get jump back entry of kernel B via the return
> address.
>
> Because the stack state is different between 1) and 2), the jump back
> entry of kernel A should distinguish them.

Yes.  Because the stack state is different we need to be careful.

However I don't see that we care how we got to the proper piece of
code.  If we don't care we don't need to distinguish them.

Therefore I see two possible solutions.
1) Write a tiny trampoline that goes in the core file to keep
   the calling conventions sane.

2) After we figure out our address read the stack pointer from
   a fixed location and simply set it.  (This is my preference)

Eric
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