[Devel] Re: [RFC][PATCH] x86_86 support of checkpoint/restart (Re: Checkpoint / Restart)

[Nauman Rafique]

On Mon, Feb 9, 2009 at 10:02 AM, Dave Hansen <dave at linux.vnet.ibm.com> wrote:
> On Fri, 2009-02-06 at 16:17 -0800, Nauman Rafique wrote:
>> The patch sent by Masahiko assumes that all the user-space registers
>> are saved on
>> the kernel stack on a system call. This is not true for the majority
>> of the system calls. The callee saved registers (as defined by x86_64
>> ABI) - rbx, rbp, r12, r13, r14, r15 - are saved only in some special
>> cases. That means that these registers would not be available to
>> checkpoint code. Moreover, the restore code would have no space in
>> stack to restore those registers.
>
> According to this:
>
> http://msdn.microsoft.com/en-us/library/6t169e9c(VS.80).aspx
>
> Those registers all get clobbered on all function calls.  I assume that
> userspace also considers them to get clobbered on system calls as
> well.
>
> What are those special cases you are talking about?  Certain special
> cases for entering the kernel where we do save those registers?

There are the system calls the use the same stub that I have used to
save the full stack (and thus all the registers).
	sys_clone
	sys_fork
	sys_vfork
	sys_sigaltstack
	sys_iopl

>
> Signal handling and ptrace single stepping are two places I would
> imagine we have to enter the kernel and preserve those registers.  Is
> that why you were suggesting overloading signal delivery?
>
> Thanks for pointing out the problem, though.  This one will be
> interesting. :)
>
> -- Dave
>
>
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