Subject: Re: [PATCH 2.6.27] [POWERPC] Invalidate all TLB entries in a specified range

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 15 Aug 2008 08:28:01 +1000

On Thu, 2008-08-14 at 07:11 -0600, Rocky Craig wrote:
> From: Rocky Craig <[email-masked]>
> 
> The apparent intent of "flush_tlbs" is to invalidate TLB entries that
> might match in the address range 0 to 0x00400000.  A loop counter is 
> set up at the high value and decremented by page size.  However, the 
> loop is only done once as the sense of the conditional branch at the
> loop end does not match the setup/decrement.
> 
> Signed-off-by: Rocky Craig <[email-masked]>
> ---
> 
> Source is from 2.6.27 development, but the bug appears as far back as 2.4.0.
> The small user-space program below demonstrates the loop behavior.  It was
> compiled via crosstool gcc 3.4.5 / glibc 2.3.6 for an MPC8347 target.

Heh nice, that's a bug as old as the port I would say :-)

Ben.
 
> int main()
> {
> 	long endval;				// 16(r31)
> 
> 	__asm__ __volatile__(
> 		"	lis 10,0x40\n"
> 		"1:	addic. 10,10,-0x1000\n"
> 		"	bgt 1b\n"
> 		"	stw 10,16(31)\n");	// endval
> 
> 	printf("end value = 0x%08lx\n", endval);
> }
> 
> This might win the prize for "Smallest actual code patch ever".
> 
> --- a/arch/powerpc/kernel/head_32.S.orig	2008-07-24 19:25:09.000000000 -0600
> +++ a/arch/powerpc/kernel/head_32.S	2008-07-24 19:25:22.000000000 -0600
> @@ -1155,7 +1155,7 @@ flush_tlbs:
>  	lis	r10, 0x40
>  1:	addic.	r10, r10, -0x1000
>  	tlbie	r10
> -	blt	1b
> +	bgt	1b
>  	sync
>  	blr
>  
> 
>