Re: [Qemu-devel] Coroutines and ucontext

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/27/2012 01:39 PM, Alex Barcelo wrote:
> I have read that one of the reasons for using makecontext is that it
> saves the signal state. But there also exist functions like
> "sigsetjmp" and "siglongjmp" which can be used to jump around the
> coroutines while preserving signal masks.
>
> I have a patch that uses sigsetjmp and siglongjmp instead of
> makecontext and getcontext (and all the ucontext stuff), and it
> *seems* to work... but I'm not sure if it works "by accident" (not
> sure what I'm doing to the stack, not sure what I should be doing to
> the stack).

You can post it, don't worry.  I'm curious how you are switching stacks 
when creating the coroutine.

> I will test more, but first I wanted to ask a little bit
> for advice and comments. (Well, I have to admit it: the only benchmark
> I have done is "qemu-img create -f qcow2 imgfile.qcow2 5G"... an
> extremely poor test, but enough to see if something works at all).

Booting a guest (even a raw image will do) is a decent smoke test.

> On a related side note, this is not very well-written:
>      /* The ucontext functions preserve signal masks which incurs a system call
>       * overhead.  setjmp()/longjmp() does not preserve signal masks but only
>       * works on the current stack.  Since we need a way to create and switch to
>       * a new stack, use the ucontext functions for that but setjmp()/longjmp()
>       * for everything else.
>       */
> [coroutine-ucontext.c, static Coroutine *coroutine_new(void)]
> Because it is not clear (IMHO) why are the exact reasons for not using
> setjmp and longjmp. Is it because the signal masks? Or is it (also?)
> because the "only works on the current stack"?

It's because you have to create a new stack for the new coroutine. 
makecontext does it for you; you can later use it with setcontext. 
Anything else (getcontext+setcontext, setjmp+longjmp, 
sigsetjmp+siglongjmp) will only work on an existing stack.

> But which system call are we talking about?

sigprocmask, which is invoked by sigsetjmp/siglongjmp and also 
getcontext/setcontext.  That's what we want to avoid.

Paolo