Re: [PATCH v6 14/17] powerpc: Add support for setting SPRN_TIDR

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2017-08-14 at 21:16 +1000, Michael Ellerman wrote:
> Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> writes:
> 
> > We need the SPRN_TIDR to bet set for use with fast thread-wakeup
> > (core-to-core wakeup).  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.
> 
> Each thread in a process already has a unique id, ie. its pid (in the
> init PID namespace), accessible in the kernel as task_pid_nr(task).
> 
> So if that's all we need, we don't need a new allocator, and we don't
> need to store it in the thread_struct.

We need an allocator, I think, due to size restriction on the HW TID.

> Also 99.99% of processes aren't going to care about the TIDR, so we
> should avoid setting it in the common case. ie. it should start out zero
> and only be initialised in the FTW code, or a helper that it calls.


> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 9f3e2c9..6123859 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -1213,6 +1213,16 @@ struct task_struct *__switch_to(struct task_struct *prev,
> >  		hard_irq_disable();
> >  	}
> >  
> > +#ifdef CONFIG_PPC_VAS
> > +	mtspr(SPRN_TIDR, new->thread.tidr);
> > +#endif
> 
> That should be in restore_sprs().
> 
> It should also check that the TIDR is initialised, and only switch it
> when necessary.
> 
> > +	/*
> > +	 * We can't take a PMU exception inside _switch() since there is a
> > +	 * window where the kernel stack SLB and the kernel stack are out
> > +	 * of sync. Hard disable here.
> > +	 */
> > +	hard_irq_disable();
> 
> We removed that in June in:
> 
>  e4c0fc5f72bc ("powerpc/64s: Leave interrupts hard enabled in context switch for radix")
> 
> You've obviously picked it up somewhere along the line during a rebase,
> please be more careful!
> 
> cheers