Re: [PATCH v2 13/18] powerpc: Add support for setting SPRN_TIDR

[Frederic Barrat <fbarrat@linux.vnet.ibm.com>]

Le 07/10/2017 à 04:28, Sukadev Bhattiprolu a écrit :
> We need the SPRN_TIDR to be set for use with fast thread-wakeup (core-
> to-core wakeup) and also with CAPI.
> 
> Each thread in a process needs to have a unique id within the process.
> But as explained below, for now, we assign globally unique thread ids
> to all threads in the system.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
> Signed-off-by: Philippe Bergheaud <felix@linux.vnet.ibm.com>
> Signed-off-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
> ---

FWIW
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>


> Changelog[v3]
> 	- Merge changes with and address comments to Christophe's patch.
> 	  (i.e drop CONFIG_PPC_VAS; use CONFIG_PPC64; check CPU_ARCH_300
> 	  before setting TIDR). Defer following to separate patches:
> 	  	- emulation parts of Christophe's patch,
> 		- setting TIDR for tasks other than 'current'
> 		- setting feature bit in AT_HWCAP2
> 
> Changelog[v2]
> 	- Michael Ellerman: Use an interface to assign TIDR so it is
> 	    assigned to only threads that need it; move assignment to
> 	    restore_sprs(). Drop lint from rebase;
> ---
>   arch/powerpc/include/asm/processor.h |   1 +
>   arch/powerpc/include/asm/switch_to.h |   3 +
>   arch/powerpc/kernel/process.c        | 122 +++++++++++++++++++++++++++++++++++
>   3 files changed, 126 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
> index fab7ff8..58cc212 100644
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -329,6 +329,7 @@ struct thread_struct {
>   	 */
>   	int		dscr_inherit;
>   	unsigned long	ppr;	/* used to save/restore SMT priority */
> +	unsigned long	tidr;
>   #endif
>   #ifdef CONFIG_PPC_BOOK3S_64
>   	unsigned long	tar;
> diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
> index 17c8380..f5da32f 100644
> --- a/arch/powerpc/include/asm/switch_to.h
> +++ b/arch/powerpc/include/asm/switch_to.h
> @@ -91,4 +91,7 @@ static inline void clear_task_ebb(struct task_struct *t)
>   #endif
>   }
> 
> +extern int set_thread_tidr(struct task_struct *t);
> +extern void clear_thread_tidr(struct task_struct *t);
> +
>   #endif /* _ASM_POWERPC_SWITCH_TO_H */
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 37ed60b..d861fcd 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1120,6 +1120,13 @@ static inline void restore_sprs(struct thread_struct *old_thread,
>   			mtspr(SPRN_TAR, new_thread->tar);
>   	}
>   #endif
> +#ifdef CONFIG_PPC64
> +	if (old_thread->tidr != new_thread->tidr) {
> +		/* TIDR should be non-zero only with ISA3.0. */
> +		WARN_ON_ONCE(!cpu_has_feature(CPU_FTR_ARCH_300));
> +		mtspr(SPRN_TIDR, new_thread->tidr);
> +	}
> +#endif
>   }
> 
>   #ifdef CONFIG_PPC_BOOK3S_64
> @@ -1438,9 +1445,117 @@ void flush_thread(void)
>   #endif /* CONFIG_HAVE_HW_BREAKPOINT */
>   }
> 
> +#ifdef CONFIG_PPC64
> +static DEFINE_SPINLOCK(vas_thread_id_lock);
> +static DEFINE_IDA(vas_thread_ida);
> +
> +/*
> + * We need to assign a unique thread id to each thread in a process.
> + *
> + * This thread id, referred to as TIDR, and separate from the Linux's tgid,
> + * is intended to be used to direct an ASB_Notify from the hardware to the
> + * thread, when a suitable event occurs in the system.
> + *
> + * One such event is a "paste" instruction in the context of Fast Thread
> + * Wakeup (aka Core-to-core wake up in the Virtual Accelerator Switchboard
> + * (VAS) in POWER9.
> + *
> + * To get a unique TIDR per process we could simply reuse task_pid_nr() but
> + * the problem is that task_pid_nr() is not yet available copy_thread() is
> + * called. Fixing that would require changing more intrusive arch-neutral
> + * code in code path in copy_process()?.
> + *
> + * Further, to assign unique TIDRs within each process, we need an atomic
> + * field (or an IDR) in task_struct, which again intrudes into the arch-
> + * neutral code. So try to assign globally unique TIDRs for now.
> + *
> + * NOTE: TIDR 0 indicates that the thread does not need a TIDR value.
> + *	 For now, only threads that expect to be notified by the VAS
> + *	 hardware need a TIDR value and we assign values > 0 for those.
> + */
> +#define MAX_THREAD_CONTEXT	((1 << 16) - 1)
> +static int assign_thread_tidr(void)
> +{
> +	int index;
> +	int err;
> +
> +again:
> +	if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL))
> +		return -ENOMEM;
> +
> +	spin_lock(&vas_thread_id_lock);
> +	err = ida_get_new_above(&vas_thread_ida, 1, &index);
> +	spin_unlock(&vas_thread_id_lock);
> +
> +	if (err == -EAGAIN)
> +		goto again;
> +	else if (err)
> +		return err;
> +
> +	if (index > MAX_THREAD_CONTEXT) {
> +		spin_lock(&vas_thread_id_lock);
> +		ida_remove(&vas_thread_ida, index);
> +		spin_unlock(&vas_thread_id_lock);
> +		return -ENOMEM;
> +	}
> +
> +	return index;
> +}
> +
> +static void free_thread_tidr(int id)
> +{
> +	spin_lock(&vas_thread_id_lock);
> +	ida_remove(&vas_thread_ida, id);
> +	spin_unlock(&vas_thread_id_lock);
> +}
> +
> +/*
> + * Clear any TIDR value assigned to this thread.
> + */
> +void clear_thread_tidr(struct task_struct *t)
> +{
> +	if (!t->thread.tidr)
> +		return;
> +
> +	if (!cpu_has_feature(CPU_FTR_ARCH_300)) {
> +		WARN_ON_ONCE(1);
> +		return;
> +	}
> +
> +	mtspr(SPRN_TIDR, 0);
> +	free_thread_tidr(t->thread.tidr);
> +	t->thread.tidr = 0;
> +}
> +
> +/*
> + * Assign a unique TIDR (thread id) for task @t and set it in the thread
> + * structure. For now, we only support setting TIDR for 'current' task.
> + */
> +int set_thread_tidr(struct task_struct *t)
> +{
> +	if (!cpu_has_feature(CPU_FTR_ARCH_300))
> +		return -EINVAL;
> +
> +	if (t != current)
> +		return -EINVAL;
> +
> +	t->thread.tidr = assign_thread_tidr();
> +	if (t->thread.tidr < 0)
> +		return t->thread.tidr;
> +
> +	mtspr(SPRN_TIDR, t->thread.tidr);
> +
> +	return 0;
> +}
> +
> +#endif /* CONFIG_PPC64 */
> +
>   void
>   release_thread(struct task_struct *t)
>   {
> +#ifdef CONFIG_PPC64
> +	clear_thread_tidr(t);
> +#endif
>   }
> 
>   /*
> @@ -1466,6 +1581,10 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> 
>   	clear_task_ebb(dst);
> 
> +#ifdef CONFIG_PPC64
> +	dst->thread.tidr = 0;
> +#endif
> +
>   	return 0;
>   }
> 
> @@ -1576,6 +1695,9 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
>   #endif
> 
>   	setup_ksp_vsid(p, sp);
> +#ifdef CONFIG_PPC64
> +	p->thread.tidr = 0;
> +#endif
> 
>   #ifdef CONFIG_PPC64
>   	if (cpu_has_feature(CPU_FTR_DSCR)) {
>