On Thu, 2 Oct 2014 16:43:41 -0700
Nishanth Aravamudan <n...@linux.vnet.ibm.com> wrote:
> 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?
>
Yes, i is the index of the 16-bit chunk we want to read. I will add
numerical examples:
chunk #0 is (u16) packed[0] >> 48
chunk #1 is (u16) packed[0] >> 32
chunk #2 is (u16) packed[0] >> 16
chunk #3 is (u16) packed[0] >> 0
chunk #4 is (u16) packed[1] >> 48
chunk #5 is (u16) packed[1] >> 32
...
chunk #22 is (u16) packed[1] >> 16
chunk #23 is (u16) packed[1] >> 0
> > +}
> > +
> > +/*
> > * 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?
>
Oops ! The u16 one is supposed to supersede the __be16 * one :)
- const __be16 *field = (const __be16 *) packed;
> > +
> > + 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?
Same as above ^^
- } else if (be16_to_cpup(field) & VPHN_FIELD_MSB) {
+ } else if (field & VPHN_FIELD_MSB) {
Bad copy/paste from the prototype code... :\
Thanks for the review.
>
> > /* 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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>
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