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

[Young Koh <young.koh@gmail.com>]

> > Yeah, this is reasonable.  You have to be careful that you save and restore
> > any registers that might be used by one of the stubs, but they don't use
> > FP.
> I also thought about not saving FP-regs on each kernel entry. But if you do
> this optimization, you need to save / restore FP-regs on switch_to. Also you
> need to get the FP-regs when setting up a signal-handler stackframe. And they
> have to be restored on sys_(rt_)sigreturn from the values found in the
> stackframe.
 
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

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)

Thanks a lot!



> As s390 can give me regs and FP-regs in one single ptrace call, I decided to
> not do this optimization. Maybe for i386 and x86_64 it would be worth the cost.
> 
>        Bodo
> >
> >                               Jeff
>


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