Re: [PATCH] kexec based hibernation: a prototype of kexecmulti-stage load

[Huang, Ying]

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. Possible solution can be as
follow:

a) Before kernel A call some physical mode code or kernel B, it set
argument 1 to be a magic number that can not be return address (such as
-1). Jump back entry of kernel A can check whether the stack top is
argument 1 or return address.

b) Distinguish by return address. Such as, called physical mode code
must return 0, while kernel B must set %eax to some other number.

c) Use different entry point for 1) and 2). Two entry points are deduced
from return address. Such as:

entry1 = return_address;
entry2 = return_address & ~0xfff;	/* page aligned */

entry1 is used by physical mode code. entry2 is used by kernel B.


Which one is better? Or some other solution?

Best Regards,
Huang Ying

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