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
