Re: [PATCH v7 17/17] spapr: Fold spapr_node0_size() into its only caller

[Greg Kurz <groug@kaod.org>]

On Tue,  3 Mar 2020 14:43:51 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> The Real Mode Area (RMA) needs to fit within the NUMA node owning memory
> at address 0.  That's usually node 0, but can be a later one if there are
> some nodes which have no memory (only CPUs).
> 
> This is currently handled by the spapr_node0_size() helper.  It has only
> one caller, so there's not a lot of point splitting it out.  It's also
> extremely easy to misread the code as clamping to the size of the smallest
> node rather than the first node with any memory.
> 
> So, fold it into the caller, and add some commentary to make it a bit
> clearer exactly what it's doing.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/ppc/spapr.c | 37 +++++++++++++++++++++----------------
>  1 file changed, 21 insertions(+), 16 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 2eb0d8f70d..d674a9f48f 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -296,20 +296,6 @@ static void spapr_populate_pa_features(SpaprMachineState *spapr,
>      _FDT((fdt_setprop(fdt, offset, "ibm,pa-features", pa_features, pa_size)));
>  }
>  
> -static hwaddr spapr_node0_size(MachineState *machine)
> -{
> -    if (machine->numa_state->num_nodes) {
> -        int i;
> -        for (i = 0; i < machine->numa_state->num_nodes; ++i) {
> -            if (machine->numa_state->nodes[i].node_mem) {
> -                return MIN(pow2floor(machine->numa_state->nodes[i].node_mem),
> -                           machine->ram_size);
> -            }
> -        }
> -    }
> -    return machine->ram_size;
> -}
> -
>  static void add_str(GString *s, const gchar *s1)
>  {
>      g_string_append_len(s, s1, strlen(s1) + 1);
> @@ -2653,10 +2639,24 @@ static hwaddr spapr_rma_size(SpaprMachineState *spapr, Error **errp)
>      MachineState *machine = MACHINE(spapr);
>      SpaprMachineClass *smc = SPAPR_MACHINE_GET_CLASS(spapr);
>      hwaddr rma_size = machine->ram_size;
> -    hwaddr node0_size = spapr_node0_size(machine);
>  
>      /* RMA has to fit in the first NUMA node */
> -    rma_size = MIN(rma_size, node0_size);
> +    if (machine->numa_state->num_nodes) {
> +        /*
> +         * It's possible for there to be some zero-memory nodes first
> +         * in the list.  We need the RMA to fit inside the memory of
> +         * the first node which actually has some memory.
> +         */
> +        int i;
> +
> +        for (i = 0; i < machine->numa_state->num_nodes; ++i) {
> +            if (machine->numa_state->nodes[i].node_mem != 0) {
> +                rma_size = MIN(rma_size,
> +                               machine->numa_state->nodes[i].node_mem);
> +                break;
> +            }
> +        }
> +    }
>  
>      /*
>       * VRMA access is via a special 1TiB SLB mapping, so the RMA can
> @@ -2673,6 +2673,11 @@ static hwaddr spapr_rma_size(SpaprMachineState *spapr, Error **errp)
>          rma_size = MIN(rma_size, smc->rma_limit);
>      }
>  
> +    /*
> +     * RMA size must be a power of 2
> +     */
> +    rma_size = pow2floor(rma_size);
> +

I saw somewhere else that the reason behind this might be
related to:

https://git.qemu.org/?p=qemu.git;a=commitdiff;h=6010818c30ce9c

commit 6010818c30ce9c796b4e22fd261fc6fea1cecbfc
Author: Alexey Kardashevskiy <aik@ozlabs.ru>
Date:   Thu Jul 3 13:10:05 2014 +1000

    spapr: Split memory nodes to power-of-two blocks

Is this the reason ?

In any case, it would probably help to mention somewhere
why the rounding is introduced by this patch.

>      if (rma_size < MIN_RMA_SLOF) {
>          error_setg(errp,
>  "pSeries SLOF firmware requires >= %ldMiB guest RMA (Real Mode Area memory)",