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Date: Wed, 26 Jan 2011 10:00:11 -0600
From: Anthony Liguori <aliguori@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] [RFC][PATCH 01/12] coroutine: Add gtk-vnc coroutines
	library
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To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

On 01/26/2011 09:25 AM, Avi Kivity wrote:
> 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.

Yeah, there are further optimizations that can be had but really, it's 
not important here.  In fact, I'd rather start with the threaded version 
just because it's far more portable than ucontext.

Regards,

Anthony Liguori