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

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Thu, 27 Nov 2014 10:39:23 +1100
Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> 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 ...
> 

My understanding of PAPR+ is that H_HOME_NODE_ASSOCIATIVITY returns a sequence of
numbers in registers R4 to R9 (that is 64 * 6 = 384 bits). The numbers are either
16-bit long (if high order bit is 1) or 32-bit long. The remaining unused bits are
set to 1. 

Of course, in a LE guest, plpar_hcall9() stores flipped values to memory.

> >  
> >  #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 ?
> 

Yes. FWIW, it is exactly what the current code does.

> > +		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 we get USED,USED,USED,UNUSED from memory, that means the hypervisor
has returned UNUSED,USED,USED,USED. My point is that it cannot happen:
why would the hypervisor care to pack a sequence of useful numbers with
holes in it ? 
FWIW, I could never observe such a thing in a PowerVM guest... All ones always
come after the payload.

> > -		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;
> 
>