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Date: Wed, 26 Jan 2011 17:25:39 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [RFC][PATCH 01/12] coroutine: Add gtk-vnc coroutines
	library
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org

On 01/22/2011 11:29 AM, Stefan Hajnoczi wrote:
> Asynchronous image format code is becoming very complex.  Let's try
> using coroutines to write sequential code without callbacks but use
> coroutines to switch stacks under the hood.
>
>
> +
> +int cc_swap(struct continuation *from, struct continuation *to)
> +{
> +	to->exited = 0;
> +	if (getcontext(&to->last) == -1)
> +		return -1;
> +	else if (to->exited == 0)
> +		to->exited = 1;
> +	else if (to->exited == 1)
> +		return 1;
> +
> +	return swapcontext(&from->uc,&to->uc);
> +}

swapcontext() is very slow, involving the fpu and a syscall.

A nice trick I've used in the past is to use getcontext/makecontext for 
the initial setup and setjmp/longjmp for switching.  Of course this can 
be done later, as an optimization.

-- 
error compiling committee.c: too many arguments to function