Re: [PATCH] spapr_numa.c: fixes in spapr_numa_FORM2_write_rtas_tables()

[Greg Kurz <groug@kaod.org>]

On Wed, 22 Sep 2021 13:17:32 +0200
Philippe Mathieu-Daudé <philmd@redhat.com> wrote:

> On 9/21/21 21:43, Daniel Henrique Barboza wrote:
> > This patch has a handful of modifications for the recent added
> > FORM2 support:
> > 
> > - there is no particular reason for both 'lookup_index_table' and
> > 'distance_table' to be allocated in the heap, since their sizes are
> > known right at the start of the function. Use static allocation in
> > them to spare a couple of g_new0() calls;
> > 
> > - to not allocate more than the necessary size in 'distance_table'. At
> > this moment the array is oversized due to allocating uint32_t for all
> > elements, when most of them fits in an uint8_t;
> > 
> > - create a NUMA_LOCAL_DISTANCE macro to avoid hardcoding the local
> > distance value.
> > 
> > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > ---
> >   hw/ppc/spapr_numa.c | 35 +++++++++++++++++++----------------
> >   1 file changed, 19 insertions(+), 16 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr_numa.c b/hw/ppc/spapr_numa.c
> > index 58d5dc7084..039a0439c6 100644
> > --- a/hw/ppc/spapr_numa.c
> > +++ b/hw/ppc/spapr_numa.c
> > @@ -19,6 +19,9 @@
> >   /* Moved from hw/ppc/spapr_pci_nvlink2.c */
> >   #define SPAPR_GPU_NUMA_ID           (cpu_to_be32(1))
> >   
> > +/* Macro to avoid hardcoding the local distance value */
> > +#define NUMA_LOCAL_DISTANCE         10
> > +
> >   /*
> >    * Retrieves max_dist_ref_points of the current NUMA affinity.
> >    */
> > @@ -500,17 +503,21 @@ static void spapr_numa_FORM2_write_rtas_tables(SpaprMachineState *spapr,
> >       MachineState *ms = MACHINE(spapr);
> >       NodeInfo *numa_info = ms->numa_state->nodes;
> >       int nb_numa_nodes = ms->numa_state->num_nodes;
> > +    /* Lookup index table has an extra uint32_t with its length */
> > +    uint32_t lookup_index_table[nb_numa_nodes + 1];
> >       int distance_table_entries = nb_numa_nodes * nb_numa_nodes;
> > -    g_autofree uint32_t *lookup_index_table = NULL;
> > -    g_autofree uint32_t *distance_table = NULL;
> > -    int src, dst, i, distance_table_size;
> > -    uint8_t *node_distances;
> 
> This should have be of ptrdiff_t type.
> 

Why ? I don't see pointer subtraction in the code.

> > +    /*
> > +     * Distance table is an uint8_t array with a leading uint32_t
> > +     * containing its length.
> > +     */
> > +    uint8_t distance_table[distance_table_entries + 4];
> 
> The previous code seems better by using the heap, now we have
> to worry about stack overflow...
> 

Indeed the size of this array could be up to 16k + 4. I guess
Philippe's point make sense. David's request was to use uint8_t
instead of uin32t_t, not to drop g_new0(). Please revert to
using the heap.

lookup_index_table[] can _only_ grow up to 516 bytes, which is
less problematic, but it doesn't hurt either to allocate it
on the heap. Not changing that would make this patch simpler.

> > +    uint32_t *distance_table_length;
> 
> Please drop, ...
> 
> > +    int src, dst, i;
> >   
> >       /*
> >        * ibm,numa-lookup-index-table: array with length and a
> >        * list of NUMA ids present in the guest.
> >        */
> > -    lookup_index_table = g_new0(uint32_t, nb_numa_nodes + 1);
> >       lookup_index_table[0] = cpu_to_be32(nb_numa_nodes);
> >   
> >       for (i = 0; i < nb_numa_nodes; i++) {
> > @@ -518,8 +525,7 @@ static void spapr_numa_FORM2_write_rtas_tables(SpaprMachineState *spapr,
> >       }
> >   
> >       _FDT(fdt_setprop(fdt, rtas, "ibm,numa-lookup-index-table",
> > -                     lookup_index_table,
> > -                     (nb_numa_nodes + 1) * sizeof(uint32_t)));
> > +                     lookup_index_table, sizeof(lookup_index_table)));
> >   
> >       /*
> >        * ibm,numa-distance-table: contains all node distances. First
> > @@ -531,11 +537,10 @@ static void spapr_numa_FORM2_write_rtas_tables(SpaprMachineState *spapr,
> >        * array because NUMA ids can be sparse (node 0 is the first,
> >        * node 8 is the second ...).
> >        */
> > -    distance_table = g_new0(uint32_t, distance_table_entries + 1);
> > -    distance_table[0] = cpu_to_be32(distance_table_entries);
> > +    distance_table_length = (uint32_t *)distance_table;
> > +    distance_table_length[0] = cpu_to_be32(distance_table_entries);
> 
> ... and use instead:
> 
>         stl_be_p(distance_table, distance_table_entries);
> 

+1

> > -    node_distances = (uint8_t *)&distance_table[1];
> > -    i = 0;
> > +    i = 4;
> >   
> >       for (src = 0; src < nb_numa_nodes; src++) {
> >           for (dst = 0; dst < nb_numa_nodes; dst++) {
> > @@ -546,18 +551,16 @@ static void spapr_numa_FORM2_write_rtas_tables(SpaprMachineState *spapr,
> >                * adding the numa_info to retrieve distance info from.
> >                */
> >               if (src == dst) {
> > -                node_distances[i++] = 10;
> > +                distance_table[i++] = NUMA_LOCAL_DISTANCE;
> >                   continue;
> >               }
> >   
> > -            node_distances[i++] = numa_info[src].distance[dst];
> > +            distance_table[i++] = numa_info[src].distance[dst];
> >           }
> >       }
> >   
> > -    distance_table_size = distance_table_entries * sizeof(uint8_t) +
> > -                          sizeof(uint32_t);
> >       _FDT(fdt_setprop(fdt, rtas, "ibm,numa-distance-table",
> > -                     distance_table, distance_table_size));
> > +                     distance_table, sizeof(distance_table)));
> >   }
> >   
> >   /*
> > 
>