Re: [PATCH] xen/x86: Be more power-efficient when waiting forever

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 02/12/13 15:03, Jan Beulich wrote:
>>>> On 02.12.13 at 15:38, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> There is one final for(;;); loop, but is in common code and immediately
>> following a call to panic(), so is actually dead code and will be cleaned up
>> in v2 of my "noreturn" series at the start of 4.5
> In the context of this, this one ...
>
>> --- a/xen/arch/x86/efi/boot.c
>> +++ b/xen/arch/x86/efi/boot.c
>> @@ -201,7 +201,8 @@ static void __init __attribute__((__noreturn__)) 
>> blexit(const CHAR16 *str)
>>          efi_bs->FreePages(xsm.addr, PFN_UP(xsm.size));
>>  
>>      efi_bs->Exit(efi_ih, EFI_SUCCESS, 0, NULL);
>> -    for( ; ; ); /* not reached */
>> +    for ( ; ; ) /* not reached */
>> +        halt();
> ... seems pointless (but of course also not wrong).

I suppose - I guess all that is needed here is for the compiler to find
no way to the end of this function.

I wonder whether an "unreachable()" would suffice?  In the optimal case,
the compiler should not emit any instructions for it.

~Andrew

>
>> --- a/xen/arch/x86/traps.c
>> +++ b/xen/arch/x86/traps.c
>> @@ -1448,7 +1448,8 @@ void __init do_early_page_fault(struct cpu_user_regs *regs)
>>          printk("Stack dump: ");
>>          while ( ((long)stk & ((PAGE_SIZE - 1) & ~(BYTES_PER_LONG - 1))) != 0 )
>>              printk("%p ", _p(*stk++));
>> -        for ( ; ; ) ;
>> +        for ( ; ; )
>> +            halt();
> While clearly this one is desirable.
>
> Jan
>