Subject: Re: [PATCH REPOST 3/3] powerpc/vphn: move endianness fixing to vphn_unpack_associativity()

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 27 Nov 2014 10:39:23 +1100

On Mon, 2014-11-17 at 18:42 +0100, Greg Kurz wrote:
> The first argument to vphn_unpack_associativity() is a const long *, but the
> parsing code expects __be64 values actually. This is inconsistent. We should
> either pass a const __be64 * or change vphn_unpack_associativity() so that
> it fixes endianness by itself.
> 
> This patch does the latter, since the caller doesn't need to know about
> endianness and this allows to fix significant 64-bit values only. Please
> note that the previous code was able to cope with 32-bit fields being split
> accross two consecutives 64-bit values. Since PAPR+ doesn't say this cannot
> happen, the behaviour was kept. It requires extra checking to know when fixing
> is needed though.

While I agree with moving the endian fixing down, the patch makes me
nervous. Note that I don't fully understand the format of what we are
parsing here so I might be wrong but ...

>  
>  #define VPHN_FIELD_UNUSED	(0xffff)
>  #define VPHN_FIELD_MSB		(0x8000)
>  #define VPHN_FIELD_MASK		(~VPHN_FIELD_MSB)
>  
> -	for (i = 1; i < VPHN_ASSOC_BUFSIZE; i++) {
> -		if (be16_to_cpup(field) == VPHN_FIELD_UNUSED)
> +	for (i = 1, j = 0, k = 0; i < VPHN_ASSOC_BUFSIZE;) {
> +		u16 field;
> +
> +		if (j % 4 == 0) {
> +			fixed.packed[k] = cpu_to_be64(packed[k]);
> +			k++;
> +		}

So we have essentially a bunch of 16-bit fields ... the above loads and
swap a whole 4 of them at once. However that means not only we byteswap
them individually, but we also flip the order of the fields. This is
ok ?

> +		field = be16_to_cpu(fixed.field[j]);
> +
> +		if (field == VPHN_FIELD_UNUSED)
>  			/* All significant fields processed.
>  			 */
>  			break;

For example, we might have USED,USED,USED,UNUSED ... after the swap, we
now have UNUSED,USED,USED,USED ... and we stop parsing in the above
line on the first one. Or am I missing something ? 

> -		if (be16_to_cpup(field) & VPHN_FIELD_MSB) {
> +		if (field & VPHN_FIELD_MSB) {
>  			/* Data is in the lower 15 bits of this field */
> -			unpacked[i] = cpu_to_be32(
> -				be16_to_cpup(field) & VPHN_FIELD_MASK);
> -			field++;
> +			unpacked[i++] = cpu_to_be32(field & VPHN_FIELD_MASK);
> +			j++;
>  		} else {
>  			/* Data is in the lower 15 bits of this field
>  			 * concatenated with the next 16 bit field
>  			 */
> -			unpacked[i] = *((__be32 *)field);
> -			field += 2;
> +			if (unlikely(j % 4 == 3)) {
> +				/* The next field is to be copied from the next
> +				 * 64-bit input value. We must fix it now.
> +				 */
> +				fixed.packed[k] = cpu_to_be64(packed[k]);
> +				k++;
> +			}
> +
> +			unpacked[i++] = *((__be32 *)&fixed.field[j]);
> +			j += 2;
>  		}
>  	}
>  
> @@ -1460,11 +1479,8 @@ static long hcall_vphn(unsigned long cpu, __be32 *associativity)
>  	long retbuf[PLPAR_HCALL9_BUFSIZE] = {0};
>  	u64 flags = 1;
>  	int hwcpu = get_hard_smp_processor_id(cpu);
> -	int i;
>  
>  	rc = plpar_hcall9(H_HOME_NODE_ASSOCIATIVITY, retbuf, flags, hwcpu);
> -	for (i = 0; i < VPHN_REGISTER_COUNT; i++)
> -		retbuf[i] = cpu_to_be64(retbuf[i]);
>  	vphn_unpack_associativity(retbuf, associativity);
>  
>  	return rc;