Re: [Qemu-devel] [PATCH v2 3/4] target/ppc: consolidate CPU device-tree id computation in helper

[Greg Kurz <groug@kaod.org>]

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On Mon, 22 May 2017 12:04:13 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, May 19, 2017 at 12:32:20PM +0200, Greg Kurz wrote:
> > For historical reasons, we compute CPU device-tree ids with a non-trivial
> > logic. This patch consolidate the logic in a single helper to be used
> > in various places where it is currently open-coded.
> > 
> > It is okay to get rid of DIV_ROUND_UP() because we're sure that the number
> > of threads per core in the guest cannot exceed the number of threads per
> > core in the host.  
> 
> However, your new logic still gives different answers in some cases.
> In particular when max_cpus is not a multiple of smp_threads.  Which
> is generally a bad idea, but allowed for older machine types for
> compatibility.   e.g. smp_threads=4, max_cpus=6 smt=8
> 
> Old logic:
> 	         DIV_ROUND_UP(6 * 8, 4)
> 	       = ⌈48 / 4⌉ = 12
> 
> New logic gives: ⌊6 / 4⌋ * 8 + (6 % 4)
>                = 1 * 8 + 2
> 	       = 10
> 

I now realize this two formulas are hardly reconcilable... this
probably means that this patch shouldn't try to consolidate the
logic in hw/ppc/spapr.c with the one in target/ppc/translate_init.c.

> In any case the DIV_ROUND_UP() isn't to handle the case where guest
> threads-per-core is bigger than host threads-per-core, it's (IIRC) for
> the case where guest threads-per-core is not a factor of host
> threads-per-core.  Again, a bad idea, but I think allowed in some old
> cases.
> 

FWIW, DIV_ROUND_UP() comes from this commit:

f303f117fec3 spapr: ensure we have at least one XICS server

but I agree that this was a bad idea...

> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  hw/ppc/spapr.c              |    6 ++----
> >  target/ppc/cpu.h            |   17 +++++++++++++++++
> >  target/ppc/translate_init.c |    3 +--
> >  3 files changed, 20 insertions(+), 6 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 75e298b4c6be..1bb05a9a6b07 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -981,7 +981,6 @@ static void *spapr_build_fdt(sPAPRMachineState *spapr,
> >      void *fdt;
> >      sPAPRPHBState *phb;
> >      char *buf;
> > -    int smt = kvmppc_smt_threads();
> >  
> >      fdt = g_malloc0(FDT_MAX_SIZE);
> >      _FDT((fdt_create_empty_tree(fdt, FDT_MAX_SIZE)));
> > @@ -1021,7 +1020,7 @@ static void *spapr_build_fdt(sPAPRMachineState *spapr,
> >      _FDT(fdt_setprop_cell(fdt, 0, "#size-cells", 2));
> >  
> >      /* /interrupt controller */
> > -    spapr_dt_xics(DIV_ROUND_UP(max_cpus * smt, smp_threads), fdt, PHANDLE_XICP);
> > +    spapr_dt_xics(ppc_cpu_dt_id_from_index(max_cpus), fdt, PHANDLE_XICP);
> >  
> >      ret = spapr_populate_memory(spapr, fdt);
> >      if (ret < 0) {
> > @@ -1977,7 +1976,6 @@ static void spapr_init_cpus(sPAPRMachineState *spapr)
> >      MachineState *machine = MACHINE(spapr);
> >      MachineClass *mc = MACHINE_GET_CLASS(machine);
> >      char *type = spapr_get_cpu_core_type(machine->cpu_model);
> > -    int smt = kvmppc_smt_threads();
> >      const CPUArchIdList *possible_cpus;
> >      int boot_cores_nr = smp_cpus / smp_threads;
> >      int i;
> > @@ -2014,7 +2012,7 @@ static void spapr_init_cpus(sPAPRMachineState *spapr)
> >              sPAPRDRConnector *drc =
> >                  spapr_dr_connector_new(OBJECT(spapr),
> >                                         SPAPR_DR_CONNECTOR_TYPE_CPU,
> > -                                       (core_id / smp_threads) * smt);
> > +                                       ppc_cpu_dt_id_from_index(core_id));
> >  
> >              qemu_register_reset(spapr_drc_reset, drc);
> >          }
> > diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
> > index 401e10e7dad8..47fe6c64698f 100644
> > --- a/target/ppc/cpu.h
> > +++ b/target/ppc/cpu.h
> > @@ -2529,4 +2529,21 @@ int ppc_get_vcpu_dt_id(PowerPCCPU *cpu);
> >  PowerPCCPU *ppc_get_vcpu_by_dt_id(int cpu_dt_id);
> >  
> >  void ppc_maybe_bswap_register(CPUPPCState *env, uint8_t *mem_buf, int len);
> > +
> > +#if !defined(CONFIG_USER_ONLY)
> > +#include "sysemu/cpus.h"
> > +#include "target/ppc/kvm_ppc.h"
> > +
> > +static inline int ppc_cpu_dt_id_from_index(int cpu_index)
> > +{
> > +    /* POWER HV support has an historical limitation that different threads
> > +     * on a single core cannot be in different guests at the same time. In
> > +     * order to allow KVM to assign guest threads to host cores accordingly,
> > +     * CPU device tree ids are spaced by the number of threads per host cores.
> > +     */
> > +    return (cpu_index / smp_threads) * kvmppc_smt_threads()
> > +        + (cpu_index % smp_threads);
> > +}
> > +#endif
> > +
> >  #endif /* PPC_CPU_H */
> > diff --git a/target/ppc/translate_init.c b/target/ppc/translate_init.c
> > index 56a0ab22cfbe..837a9a496a65 100644
> > --- a/target/ppc/translate_init.c
> > +++ b/target/ppc/translate_init.c
> > @@ -9851,8 +9851,7 @@ static void ppc_cpu_realizefn(DeviceState *dev, Error **errp)
> >      }
> >  
> >  #if !defined(CONFIG_USER_ONLY)
> > -    cpu->cpu_dt_id = (cs->cpu_index / smp_threads) * max_smt
> > -        + (cs->cpu_index % smp_threads);
> > +    cpu->cpu_dt_id = ppc_cpu_dt_id_from_index(cs->cpu_index);
> >  
> >      if (kvm_enabled() && !kvm_vcpu_id_is_valid(cpu->cpu_dt_id)) {
> >          error_setg(errp, "Can't create CPU with id %d in KVM", cpu->cpu_dt_id);
> >   
> 


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