Re: [uml-devel] Explaination of system call function flow in TT mode

[Jeff Dike <jdike@addtoit.com>]

On Wed, May 18, 2005 at 09:24:07AM -0400, Young Koh wrote:
> Thanks for the replies. let me understand them. In a SKAS mode UML
> kernel, the pseudo code of a system call invocation would be like the
> following.
> 
> wait4(); // wait until a user process raises a syscall
> save_registers(); // copy the user process' registers to UML kernel space
> execute_syscall(); // execute the syscall in UML kernel context
> restore_registers(); // copy the user registers back to the user process

Correct.

> but, during execute_syscall(), even if its stubs use all the registers
> and/or it happens to invoke switch_to(), it will happen all in the UML
> kernel's context. that means the user process's context will be
> protected by host kernel's context switching mechanism. doesn't it?
> because the host kernel will automatically save/restore a process'
> registers when the process is stopped and resumed, all we care about
> in the above routine should be to get system call parameters from the
> tracee and save the return value to it. shouldn't it? (again, only for
> SKAS)

No, because (in skas mode) there is only one host process for all the UML
processes.  So the process registers have to be saved and restored across a 
context switch.

				Jeff


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