Re: [PATCH 1/1] spapr.c: remove 'ibm,chip-id' from DT

[Greg Kurz]

On Thu, 11 Mar 2021 12:15:57 -0300
Daniel Henrique Barboza <danielhb413@gmail.com> wrote:

> The attribute 'ibm,chip-id' does not exist in PAPR. This alone would be
> enough reason to remove it from the spapr DT, but before doing that,
> let's give a brief context on how and why it was introduced.
> 
> 'ibm,chip-id' was added in the spapr DT by commit 10582ff83279. This
> commit references kernel commit 256f2d4b463d ("powerpc: Use ibm, chip-id
> property to compute cpu_core_mask if available"). In this kernel commit,
> the 'ibm,chip-id' DT attribute is being used to calculate the
> cpu_core_mask in traverse_siblings_chip_id(). This function still need
> to consider 'ibm,chip-id' not being available as well to avoid breaking
> older guests.
> 
> Later on, kernel commit df52f6714071 ("powerpc/smp: Rework CPU topology
> construction") removed traverse_siblings_chip_id() and its callers,
> making the CPU topology calculation independent of the 'ibm,chip-id'
> attribute. Today, the kernel uses 'ibm,chip-id' for PowerNV related code
> only - the pseries kernel does not rely on it.
> 

Thanks for the archaeology ! So this means that the pseries kernel used
to rely on it up to kernel v4.14, right ?

> All that said, since it's not in PAPR and the pseries kernel does not
> rely on it, let's remove ibm,chip-id from the DT.
> 

Even if it isn't part of PAPR, this is something that we've been
exposing to guests with existing machine types and someone could
have found a use for it or still using a pre-4.14 kernel, e.g.
debian stretch still relies on 4.9.

In theory we should change this behavior in the default machine only.
But in case David is okay to merge the patch anyway,

Reviewed-by: Greg Kurz <groug@kaod.org>

> CC: Alexey Kardashevskiy <aik@ozlabs.ru>
> Suggested-by: Cédric Le Goater <clg@kaod.org>
> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> ---
>  hw/ppc/spapr.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 85fe65f894..d2e448fd9c 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -657,7 +657,6 @@ static void spapr_dt_cpu(CPUState *cs, void *fdt, int 
> offset,
>      uint32_t page_sizes_prop[64];
>      size_t page_sizes_prop_size;
>      unsigned int smp_threads = ms->smp.threads;
> -    uint32_t vcpus_per_socket = smp_threads * ms->smp.cores;
>      uint32_t pft_size_prop[] = {0, cpu_to_be32(spapr->htab_shift)};
>      int compat_smt = MIN(smp_threads, ppc_compat_max_vthreads(cpu));
>      SpaprDrc *drc;
> @@ -744,9 +743,6 @@ static void spapr_dt_cpu(CPUState *cs, void *fdt, int 
> offset,
>  
>      spapr_dt_pa_features(spapr, cpu, fdt, offset);
>  
> -    _FDT((fdt_setprop_cell(fdt, offset, "ibm,chip-id",
> -                           cs->cpu_index / vcpus_per_socket)));
> -
>      _FDT((fdt_setprop(fdt, offset, "ibm,pft-size",
>                        pft_size_prop, sizeof(pft_size_prop))));
>