On Mon, 22 May 2017 12:04:13 +1000 David Gibson <da...@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 <gr...@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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