Re: [PATCH v8 05/10] target/ppc: enable PMU counter overflow with cycle events

[David Gibson]

On Thu, Nov 25, 2021 at 12:08:12PM -0300, Daniel Henrique Barboza wrote:
65;6601;1c> The PowerISA v3.1 defines that if the proper bits are set 
(MMCR0_PMC1CE
> for PMC1 and MMCR0_PMCjCE for the remaining PMCs), counter negative
> conditions are enabled. This means that if the counter value overflows
> (i.e. exceeds 0x80000000) a performance monitor alert will occur. This alert
> can trigger an event-based exception (to be implemented in the next patches)
> if the MMCR0_EBE bit is set.
> 
> For now, overflowing the counter when the PMC is counting cycles will
> just trigger a performance monitor alert. This is done by starting the
> overflow timer to expire in the moment the overflow would be occuring. The
> timer will call fire_PMC_interrupt() (via cpu_ppc_pmu_timer_cb) which will
> trigger the PMU alert and, if the conditions are met, an EBB exception.
> 
> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

A couple of minor nits noted below, though.

> ---
>  target/ppc/cpu.h        |  2 ++
>  target/ppc/power8-pmu.c | 80 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 82 insertions(+)
> 
> diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
> index 9c732953f0..9b41b022e2 100644
> --- a/target/ppc/cpu.h
> +++ b/target/ppc/cpu.h
> @@ -364,6 +364,8 @@ typedef enum {
>  #define MMCR0_PMCC   PPC_BITMASK(44, 45) /* PMC Control */
>  #define MMCR0_FC14   PPC_BIT(58)         /* PMC Freeze Counters 1-4 bit */
>  #define MMCR0_FC56   PPC_BIT(59)         /* PMC Freeze Counters 5-6 bit */
> +#define MMCR0_PMC1CE PPC_BIT(48)         /* MMCR0 PMC1 Condition Enabled */
> +#define MMCR0_PMCjCE PPC_BIT(49)         /* MMCR0 PMCj Condition Enabled */
>  /* MMCR0 userspace r/w mask */
>  #define MMCR0_UREG_MASK (MMCR0_FC | MMCR0_PMAO | MMCR0_PMAE)
>  /* MMCR2 userspace r/w mask */
> diff --git a/target/ppc/power8-pmu.c b/target/ppc/power8-pmu.c
> index acdaee7459..01e0b9b8fc 100644
> --- a/target/ppc/power8-pmu.c
> +++ b/target/ppc/power8-pmu.c
> @@ -23,6 +23,8 @@
>  
>  #if defined(TARGET_PPC64) && !defined(CONFIG_USER_ONLY)
>  
> +#define PMC_COUNTER_NEGATIVE_VAL 0x80000000UL
> +
>  static bool pmc_is_inactive(CPUPPCState *env, int sprn)
>  {
>      if (env->spr[SPR_POWER_MMCR0] & MMCR0_FC) {
> @@ -36,6 +38,15 @@ static bool pmc_is_inactive(CPUPPCState *env, int sprn)
>      return env->spr[SPR_POWER_MMCR0] & MMCR0_FC56;
>  }
>  
> +static bool pmc_has_overflow_enabled(CPUPPCState *env, int sprn)
> +{
> +    if (sprn == SPR_POWER_PMC1) {
> +        return env->spr[SPR_POWER_MMCR0] & MMCR0_PMC1CE;
> +    }
> +
> +    return env->spr[SPR_POWER_MMCR0] & MMCR0_PMCjCE;
> +}
> +
>  /*
>   * For PMCs 1-4, IBM POWER chips has support for an implementation
>   * dependent event, 0x1E, that enables cycle counting. The Linux kernel
> @@ -123,6 +134,70 @@ static void pmu_update_cycles(CPUPPCState *env)
>      env->pmu_base_time = now;
>  }
>  
> +/*
> + * Helper function to retrieve the cycle overflow timer of the
> + * 'sprn' counter. Given that PMC5 doesn't have a timer, the
> + * amount of timers is less than the total counters and the PMC6
> + * timer is the last of the array.
> + */
> +static QEMUTimer *get_cyc_overflow_timer(CPUPPCState *env, int sprn)
> +{
> +    if (sprn == SPR_POWER_PMC5) {
> +        return NULL;

Given that the entries in the pmu_cyc_overflow_timers are just
pointers, it would probably be slightly cheaper in terms of both time
and space to just have an always-NULL entry for PMC5, rather than
having to special case it.

> +    }
> +
> +    if (sprn == SPR_POWER_PMC6) {
> +        return env->pmu_cyc_overflow_timers[PMU_TIMERS_NUM - 1];
> +    }
> +
> +    return env->pmu_cyc_overflow_timers[sprn - SPR_POWER_PMC1];
> +}
> +
> +static void pmc_update_overflow_timer(CPUPPCState *env, int sprn)
> +{
> +    QEMUTimer *pmc_overflow_timer;
> +    int64_t timeout;
> +
> +    /* PMC5 does not have an overflow timer */
> +    if (sprn == SPR_POWER_PMC5) {
> +        return;

Since you've already handled the PMC5 case in
get_cyc_overflow_timer(), you could replace this handling with just an
if (!pmc_overflow_timer) {return;}

> +    }
> +
> +    pmc_overflow_timer = get_cyc_overflow_timer(env, sprn);
> +
> +    if (pmc_get_event(env, sprn) != PMU_EVENT_CYCLES ||
> +        !pmc_has_overflow_enabled(env, sprn)) {
> +        /* Overflow timer is not needed for this counter */
> +        timer_del(pmc_overflow_timer);
> +        return;
> +    }
> +
> +    if (env->spr[sprn] >= PMC_COUNTER_NEGATIVE_VAL) {
> +        timeout =  0;
> +    } else {
> +        timeout = PMC_COUNTER_NEGATIVE_VAL - env->spr[sprn];
> +    }
> +
> +    /*
> +     * Use timer_mod_anticipate() because an overflow timer might
> +     * be already running for this PMC.
> +     */
> +    timer_mod_anticipate(pmc_overflow_timer, env->pmu_base_time + timeout);
> +}
> +
> +static void pmu_update_overflow_timers(CPUPPCState *env)
> +{
> +    int sprn;
> +
> +    /*
> +     * Scroll through all PMCs and start counter overflow timers for
> +     * PM_CYC events, if needed.
> +     */
> +    for (sprn = SPR_POWER_PMC1; sprn <= SPR_POWER_PMC6; sprn++) {
> +        pmc_update_overflow_timer(env, sprn);
> +    }
> +}
> +
>  void helper_store_mmcr0(CPUPPCState *env, target_ulong value)
>  {
>      target_ulong curr_value = env->spr[SPR_POWER_MMCR0];
> @@ -143,6 +218,9 @@ void helper_store_mmcr0(CPUPPCState *env, target_ulong 
> value)
>          (curr_FC != new_FC)) {
>          hreg_compute_hflags(env);
>      }
> +
> +    /* Update cycle overflow timers with the current MMCR0 state */
> +    pmu_update_overflow_timers(env);
>  }
>  
>  void helper_store_mmcr1(CPUPPCState *env, uint64_t value)
> @@ -164,6 +242,8 @@ void helper_store_pmc(CPUPPCState *env, uint32_t sprn, 
> uint64_t value)
>      pmu_update_cycles(env);
>  
>      env->spr[sprn] = value;
> +
> +    pmc_update_overflow_timer(env, sprn);
>  }
>  
>  static void fire_PMC_interrupt(PowerPCCPU *cpu)

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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