Subject: Re: [RFC PATCH 10/12] [WIP] powerpc/tm: Correctly save/restore checkpointed sprs

From: Cyril Bur <[email-masked]>
Date: Tue, 20 Feb 2018 14:59:29 +1100

On Tue, 2018-02-20 at 14:00 +1100, Michael Neuling wrote:
> This needs a description of what you're trying to do.  "Correctly" doesn't
> really mean anything.
> 
> 
> On Tue, 2018-02-20 at 11:22 +1100, Cyril Bur wrote:
> > ---
> >  arch/powerpc/kernel/process.c | 57 +++++++++++++++++++++++++++++++++++++++++-
> > -
> >  arch/powerpc/kernel/ptrace.c  |  9 +++----
> >  2 files changed, 58 insertions(+), 8 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index cd3ae80a6878..674f75c56172 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -859,6 +859,8 @@ static inline bool tm_enabled(struct task_struct *tsk)
> >  	return tsk && tsk->thread.regs && (tsk->thread.regs->msr & MSR_TM);
> >  }
> >  
> > +static inline void save_sprs(struct thread_struct *t);
> > +
> >  static void tm_reclaim_thread(struct thread_struct *thr, uint8_t cause)
> >  {
> >  	/*
> > @@ -879,6 +881,8 @@ static void tm_reclaim_thread(struct thread_struct *thr,
> > uint8_t cause)
> >  	if (!MSR_TM_SUSPENDED(mfmsr()))
> >  		return;
> >  
> > +	save_sprs(thr);
> > +
> >  	giveup_all(container_of(thr, struct task_struct, thread));
> >  
> >  	tm_reclaim(thr, cause);
> > @@ -991,6 +995,37 @@ void tm_recheckpoint(struct thread_struct *thread)
> >  
> >  	__tm_recheckpoint(thread);
> >  
> > +	/*
> > +	 * This is a stripped down restore_sprs(), we need to do this
> > +	 * now as we might go straight out to userspace and currently
> > +	 * the checkpointed values are on the CPU.
> > +	 *
> > +	 * TODO: Improve
> > +	 */
> > +#ifdef CONFIG_ALTIVEC
> > +	if (cpu_has_feature(CPU_FTR_ALTIVEC))
> > +		mtspr(SPRN_VRSAVE, thread->vrsave);
> > +#endif
> > +#ifdef CONFIG_PPC_BOOK3S_64
> > +	if (cpu_has_feature(CPU_FTR_DSCR)) {
> > +		u64 dscr = get_paca()->dscr_default;
> > +		if (thread->dscr_inherit)
> > +			dscr = thread->dscr;
> > +
> > +		mtspr(SPRN_DSCR, dscr);
> > +	}
> > +
> > +	if (cpu_has_feature(CPU_FTR_ARCH_207S)) {
> > +		/* The EBB regs aren't checkpointed */
> > +		mtspr(SPRN_FSCR, thread->fscr);
> > +
> > +		mtspr(SPRN_TAR, thread->tar);
> > +	}
> > +
> > +	/* I think we don't need to */
> > +	if (cpu_has_feature(CPU_FTR_ARCH_300))
> > +		mtspr(SPRN_TIDR, thread->tidr);
> > +#endif
> 
> Why are you touching all the above hunk?

I copied restore_sprs. I'm tidying that up now - we can't call
restore_sprs because we don't have a prev and next thread.

> 
> >  	local_irq_restore(flags);
> >  }
> >  
> > @@ -1193,6 +1228,11 @@ struct task_struct *__switch_to(struct task_struct
> > *prev,
> >  #endif
> >  
> >  	new_thread = &new->thread;
> > +	/*
> > +	 * Why not &prev->thread; ?
> > +	 * What is the difference between &prev->thread and
> > +	 * &current->thread ?
> > +	 */
> 
> Why not just work it out and FIX THE CODE, rather than just rabbiting on about
> it! :-P

Agreed - I started to and then had a mini freakout that things would
end really badly if they're not the same. So I left that comment as a
reminder to investigate.

They should be the same though right?

> 
> >  	old_thread = &current->thread;
> >  
> >  	WARN_ON(!irqs_disabled());
> > @@ -1237,8 +1277,16 @@ struct task_struct *__switch_to(struct task_struct
> > *prev,
> >  	/*
> >  	 * We need to save SPRs before treclaim/trecheckpoint as these will
> >  	 * change a number of them.
> > +	 *
> > +	 * Because we're now reclaiming on kernel entry, we've had to
> > +	 * already save them. Don't do it again.
> > +	 * Note: To deliver a signal in the signal context, we'll have
> > +	 * turned off TM because we don't want the signal context to
> > +	 * have the transactional state of the main thread - what if
> > +	 * we go through switch to at that point? Can we?
> >  	 */
> > -	save_sprs(&prev->thread);
> > +	if (!prev->thread.regs || !MSR_TM_ACTIVE(prev->thread.regs->msr))
> > +		save_sprs(&prev->thread);
> >  
> >  	/* Save FPU, Altivec, VSX and SPE state */
> >  	giveup_all(prev);
> > @@ -1260,8 +1308,13 @@ struct task_struct *__switch_to(struct task_struct
> > *prev,
> >  	 * for this is we manually create a stack frame for new tasks that
> >  	 * directly returns through ret_from_fork() or
> >  	 * ret_from_kernel_thread(). See copy_thread() for details.
> > +	 *
> > +	 * It isn't stricly nessesary that we avoid the restore here
> > +	 * because we'll simply restore again after the recheckpoint,
> > +	 * but we can avoid it for performance reasons.
> >  	 */
> > -	restore_sprs(old_thread, new_thread);
> > +	if (!new_thread->regs || !MSR_TM_ACTIVE(new_thread->regs->msr))
> > +		restore_sprs(old_thread, new_thread);
> >  
> >  	last = _switch(old_thread, new_thread);
> >  
> > diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> > index ca72d7391d40..16001987ba71 100644
> > --- a/arch/powerpc/kernel/ptrace.c
> > +++ b/arch/powerpc/kernel/ptrace.c
> > @@ -135,12 +135,9 @@ static void flush_tmregs_to_thread(struct task_struct
> > *tsk)
> >  	if ((!cpu_has_feature(CPU_FTR_TM)) || (tsk != current))
> >  		return;
> >  
> > -	if (MSR_TM_SUSPENDED(mfmsr())) {
> > -		tm_reclaim_current(TM_CAUSE_SIGNAL);
> > -	} else {
> > -		tm_enable();
> > -		tm_save_sprs(&(tsk->thread));
> > -	}
> > +	BUG_ON(MSR_TM_SUSPENDED(mfmsr()));
> > +	tm_enable();
> > +	tm_save_sprs(&(tsk->thread));
> >  }
> >  #else
> >  static inline void flush_tmregs_to_thread(struct task_struct *tsk) { }