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Date: Wed, 07 Apr 2004 12:24:12 +0400
From: Brad Campbell <brad@wasp.net.au>
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Subject: Re: [Qemu-devel] Qemu workstation
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To: qemu-devel@nongnu.org

Filip Navara wrote:
> Brad Campbell wrote:
> 
>>How does that work then?
> 
> 
> I don't know the exact details.
> 
> 
>>I have some code that uses a NOP loop for accurate
>>timing? That spins at 100% cpu usage, how does a 
>>NOP tell the processor to idle? HLT does.
> 
> 
> HLT instruction halts the CPU so no more instructions are
> processed and the CPU freezes. That's usable only in situation
> like Windows blue screens.

It does? My information tells me that it halts the processor until an interrupt or other wakeup 
source occurs. Check arch/i386/kernel/process.c

/*
  * We use this if we don't have any better
  * idle routine..
  */
void default_idle(void)
{
         if (current_cpu_data.hlt_works_ok && !hlt_counter) {
                 __cli();
                 if (!current->need_resched)
                         safe_halt();
                 else
                         __sti();
         }
}

and in include/asm/system.h

system.h:#define safe_halt()            __asm__ __volatile__("sti; hlt": : :"memory")

Looks like a hlt to me and not a nop in site.
The kernel does a check at boot time to see if the processor supports the hlt instruction and if it 
does it uses that in the idle loop.

Am I wrong?

Brad