I am trying to implement the idea that Nadav proposed to save memory when 
implementing callstack switch during syscall. Here is excerpt from our 


> I was also wondering that existing implementation of syscall stack makes 
> every app thread allocate it upon creation eagerly whether syscall are 
> going to be made or not. For example java apps (which I believe use libc 
> functions) would be penalized by memory usage. I was wondering how 
> difficult it would be to make allocation of syscall stack lazily only when 
> first syscall is made.

The problem is that we won't have a stack to do this allocation work on.

One thing we could do have for all threads a tiny syscall stack (doesn't 
even need to be a whole page, can be smaller!), and the first thing a 
syscall does is
to check if this thread has this tiny stack, and if it does it allocates a 
different one. The stack needs to be just big enough for malloc() to run. 
We could
even avoid calling malloc() directly and just send a message to a different 
thread to do this for us, but I think this will be an overkill.

So I am trying to avoid messing with assembly as much as possible. Is it 
possible to implement it almost all in C by calling a function from 
linux.cc/syscall_wrapper() that would lazily allocate larger stack? 
Obviously we would still need to change RSP register to new value and then 
restore it back with could be done with asm("...") instruction. Is it 

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