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

[Bodo Stroesser <bstroesser@fujitsu-siemens.com>]

Jeff Dike wrote:
> On Tue, May 17, 2005 at 01:56:55PM -0400, Young Koh wrote:
> 
>>the tracer (UML kernel) saves and restores the tracee's registers
>>using ptrace() in order to get syscall paramaters and save the return
>>value. while doing this, the tracer would need to care about only six
>>registers(eax, ebx, ecx, edx, esi, and edi) used for the syscall
>>parameter passing. I think, the tracer doesn't have to save all the
>>registers of the tracee, because the tracer will execute the syscall
>>for the tracee and the context of tracee will not be affected.
> 
> 
> This is true, but it's easier to get all of the GP registers than it is to
> selectively grab the syscall-related ones.
> 
> 
>>but, in arch/um/kernel/skas/process.c, move_registers() function gets
>>and sets even floating point registers, which i don't think will be
>>affected by the system call execution.
>>i tested and ran a UML kernel with the second ptrace() (used for
>>floating point registers) commented out, it seems to work. (probably
>>only for SKAS mode)
> 
> 
> 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.
I hope, I didn't miss some other places that would need adaption.

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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