From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH] xen/x86: Be more power-efficient when
	waiting forever
Date: Mon, 2 Dec 2013 15:06:40 +0000
Message-ID: <529CA200.5030207@citrix.com>
References: <1385995089-440-1-git-send-email-andrew.cooper3@citrix.com>
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To: Jan Beulich <JBeulich@suse.com>
Cc: George Dunlap <george.dunlap@eu.citrix.com>, Keir Fraser <keir@xen.org>, Xen-devel <xen-devel@lists.xen.org>
List-Id: xen-devel@lists.xenproject.org

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
>