On 02.10.2014 [23:59:15 +0200], Greg Kurz wrote:
> The associativity domain numbers are obtained from the hypervisor through
> registers and written into memory by the guest: the packed array passed to
> vphn_unpack_associativity() is then native-endian, unlike what was assumed
> in the following commit:
>
> commit b08a2a12e44eaec5024b2b969f4fcb98169d1ca3
> Author: Alistair Popple <alist...@popple.id.au>
> Date: Wed Aug 7 02:01:44 2013 +1000
>
> powerpc: Make NUMA device node code endian safe
>
> If a CPU home node changes, the topology gets filled with
> bogus values. This leads to severe performance breakdowns.
>
> This patch does two things:
> - extract values from the packed array with shifts, in order to be endian
> neutral
> - convert the resulting values to be32 as expected
>
> Suggested-by: Anton Blanchard <an...@samba.org>
> Signed-off-by: Greg Kurz <gk...@linux.vnet.ibm.com>
> ---
>
> I could test this code in a userland program and obtain the same
> result in little and big endian. If the 64-bit packed values
> are:
>
> 0x8001ffffffff0000
> 0x112200003344ffff
> 0xffff000055660000
> 0x7788000099aaffff
> 0xffff8002ffffffff
> 0xffffffffffffffff
>
> then the unpacked array is:
>
> 0x00000007
> 0x00000001
> 0xffffffff
> 0x00001122
> 0x00003344
> 0xffffffff
> 0x00005566
> 0x00007788
> 0x000099aa
> 0xffffffff
> 0x00000002
> 0xffffffff
> 0xffffffff
>
> I could not test in a PowerVM guest though, hence the RFC.
>
> --
> Greg
>
> arch/powerpc/mm/numa.c | 50
> ++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 40 insertions(+), 10 deletions(-)
>
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index b835bf0..06af179 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1369,38 +1369,68 @@ static int update_cpu_associativity_changes_mask(void)
> #define VPHN_ASSOC_BUFSIZE (6*sizeof(u64)/sizeof(u32) + 1)
>
> /*
> + * The associativity values are either 16-bit (VPHN_FIELD_MSB) or 32-bit
> (data
> + * or VPHN_FIELD_UNUSED). We hence need to parse the packed array into 16-bit
> + * chunks. Let's do that with bit shifts to be endian neutral.
> + *
> + * --- 16-bit chunks -->
> + * _________________________
> + * | 0 | 1 | 2 | 3 | packed[0]
> + * -------------------------
> + * _________________________
> + * | 4 | 5 | 6 | 7 | packed[1]
> + * -------------------------
> + * ...
> + * _________________________
> + * | 20 | 21 | 22 | 23 | packed[5]
> + * -------------------------
> + *
> + * >>48 >>32 >>16 >>0 <-- needed bit shift
> + *
> + * We only care for the 2 lower bits of the chunk index to compute the shift.
> + */
> +static inline u16 read_vphn_chunk(const long *packed, unsigned int i)
> +{
> + return packed[i >> 2] >> ((~i & 3) << 4);
This is some excellent magic and the comment *should* be sufficient, but
maybe an example would be good? i is the index we want to read from in
the packed array?
> +}
> +
> +/*
> * Convert the associativity domain numbers returned from the hypervisor
> * to the sequence they would appear in the ibm,associativity property.
> */
> static int vphn_unpack_associativity(const long *packed, __be32 *unpacked)
> {
> - int i, nr_assoc_doms = 0;
> + unsigned int i, j, nr_assoc_doms = 0;
> const __be16 *field = (const __be16 *) packed;
>
> #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; i < VPHN_ASSOC_BUFSIZE; i++) {
> + u16 field = read_vphn_chunk(packed, j);
Maybe I'm super dense here, but isn't there are already a variable named
field in this context? With a different annotation?
> +
> + if (field == VPHN_FIELD_UNUSED) {
> /* All significant fields processed, and remaining
> * fields contain the reserved value of all 1's.
> * Just store them.
> */
> - unpacked[i] = *((__be32 *)field);
> - field += 2;
> + unpacked[i] = (VPHN_FIELD_UNUSED << 16 |
> + VPHN_FIELD_UNUSED);
> + j += 2;
> } else if (be16_to_cpup(field) & VPHN_FIELD_MSB) {
Why do we need to do be16_to_cpup(field) here if field is now a u16? Or
more explicitly, if we don't performe be16_to_cpup(field) anywhere else
in this function now, why do we need to here?
> /* 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++;
> nr_assoc_doms++;
> } else {
> /* Data is in the lower 15 bits of this field
> * concatenated with the next 16 bit field
> */
> - unpacked[i] = *((__be32 *)field);
> - field += 2;
> + unpacked[i] =
> + cpu_to_be32((u32) field << 16 |
> + read_vphn_chunk(packed, j + 1));
> + j += 2;
> nr_assoc_doms++;
> }
> }
>
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