On Thu, 2014-11-27 at 10:28 +0100, Greg Kurz wrote:

On Thu, 27 Nov 2014 10:39:23 +1100
Benjamin Herrenschmidt [off-list ref] wrote:

quoted

On Mon, 2014-11-17 at 18:42 +0100, Greg Kurz wrote:

quoted

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. 

Ok, that's the bit I was missing. What we get is thus not a memory array
but a register one, which we "incorrectly" swap when writing to memory
inside plpar_hcall9().

Now, I'm not sure that replacing:

-	for (i = 0; i < VPHN_REGISTER_COUNT; i++)
-		retbuf[i] = cpu_to_be64(retbuf[i]);

With:

+		if (j % 4 == 0) {
+			fixed.packed[k] = cpu_to_be64(packed[k]);
+			k++;
+		}

Brings any benefit in term of readability. It makes sense to have a
"first pass" that undoes the helper swapping to re-create the original
"byte stream".

In a second pass, we parse that stream, one 16-bytes at a time, and
we could do so with a simple loop of be16_to_cpup(foo++). I wouldn't
bother with the cast to 32-bit etc... if you encounter a 32-bit case,
you just fetch another 16-bit and do value = (old << 16) | new

I think that should lead to something more readable, no ?

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

quoted

quoted

 
 #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.

quoted

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+		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.

quoted

quoted

-		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;