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Date:   Wed, 30 Jun 2021 22:48:41 -0500
From:   "Christopher M. Riedl" <cmr@...ux.ibm.com>
To:     "Daniel Axtens" <dja@...ens.net>, <linuxppc-dev@...ts.ozlabs.org>
Cc:     <tglx@...utronix.de>, <x86@...nel.org>,
        <linux-hardening@...r.kernel.org>, <keescook@...omium.org>
Subject: Re: [RESEND PATCH v4 05/11] powerpc/64s: Add ability to skip SLB
 preload

On Sun Jun 20, 2021 at 10:13 PM CDT, Daniel Axtens wrote:
> "Christopher M. Riedl" <cmr@...ux.ibm.com> writes:
>
> > Switching to a different mm with Hash translation causes SLB entries to
> > be preloaded from the current thread_info. This reduces SLB faults, for
> > example when threads share a common mm but operate on different address
> > ranges.
> >
> > Preloading entries from the thread_info struct may not always be
> > appropriate - such as when switching to a temporary mm. Introduce a new
> > boolean in mm_context_t to skip the SLB preload entirely. Also move the
> > SLB preload code into a separate function since switch_slb() is already
> > quite long. The default behavior (preloading SLB entries from the
> > current thread_info struct) remains unchanged.
> >
> > Signed-off-by: Christopher M. Riedl <cmr@...ux.ibm.com>
> >
> > ---
> >
> > v4:  * New to series.
> > ---
> >  arch/powerpc/include/asm/book3s/64/mmu.h |  3 ++
> >  arch/powerpc/include/asm/mmu_context.h   | 13 ++++++
> >  arch/powerpc/mm/book3s64/mmu_context.c   |  2 +
> >  arch/powerpc/mm/book3s64/slb.c           | 56 ++++++++++++++----------
> >  4 files changed, 50 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
> > index eace8c3f7b0a1..b23a9dcdee5af 100644
> > --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> > +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> > @@ -130,6 +130,9 @@ typedef struct {
> >  	u32 pkey_allocation_map;
> >  	s16 execute_only_pkey; /* key holding execute-only protection */
> >  #endif
> > +
> > +	/* Do not preload SLB entries from thread_info during switch_slb() */
> > +	bool skip_slb_preload;
> >  } mm_context_t;
> >  
> >  static inline u16 mm_ctx_user_psize(mm_context_t *ctx)
> > diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> > index 4bc45d3ed8b0e..264787e90b1a1 100644
> > --- a/arch/powerpc/include/asm/mmu_context.h
> > +++ b/arch/powerpc/include/asm/mmu_context.h
> > @@ -298,6 +298,19 @@ static inline int arch_dup_mmap(struct mm_struct *oldmm,
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_PPC_BOOK3S_64
> > +
> > +static inline void skip_slb_preload_mm(struct mm_struct *mm)
> > +{
> > +	mm->context.skip_slb_preload = true;
> > +}
> > +
> > +#else
> > +
> > +static inline void skip_slb_preload_mm(struct mm_struct *mm) {}
> > +
> > +#endif /* CONFIG_PPC_BOOK3S_64 */
> > +
> >  #include <asm-generic/mmu_context.h>
> >  
> >  #endif /* __KERNEL__ */
> > diff --git a/arch/powerpc/mm/book3s64/mmu_context.c b/arch/powerpc/mm/book3s64/mmu_context.c
> > index c10fc8a72fb37..3479910264c59 100644
> > --- a/arch/powerpc/mm/book3s64/mmu_context.c
> > +++ b/arch/powerpc/mm/book3s64/mmu_context.c
> > @@ -202,6 +202,8 @@ int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
> >  	atomic_set(&mm->context.active_cpus, 0);
> >  	atomic_set(&mm->context.copros, 0);
> >  
> > +	mm->context.skip_slb_preload = false;
> > +
> >  	return 0;
> >  }
> >  
> > diff --git a/arch/powerpc/mm/book3s64/slb.c b/arch/powerpc/mm/book3s64/slb.c
> > index c91bd85eb90e3..da0836cb855af 100644
> > --- a/arch/powerpc/mm/book3s64/slb.c
> > +++ b/arch/powerpc/mm/book3s64/slb.c
> > @@ -441,10 +441,39 @@ static void slb_cache_slbie_user(unsigned int index)
> >  	asm volatile("slbie %0" : : "r" (slbie_data));
> >  }
> >  
> > +static void preload_slb_entries(struct task_struct *tsk, struct mm_struct *mm)
> Should this be explicitly inline or even __always_inline? I'm thinking
> switch_slb is probably a fairly hot path on hash?

Yes absolutely. I'll make this change in v5.

>
> > +{
> > +	struct thread_info *ti = task_thread_info(tsk);
> > +	unsigned char i;
> > +
> > +	/*
> > +	 * We gradually age out SLBs after a number of context switches to
> > +	 * reduce reload overhead of unused entries (like we do with FP/VEC
> > +	 * reload). Each time we wrap 256 switches, take an entry out of the
> > +	 * SLB preload cache.
> > +	 */
> > +	tsk->thread.load_slb++;
> > +	if (!tsk->thread.load_slb) {
> > +		unsigned long pc = KSTK_EIP(tsk);
> > +
> > +		preload_age(ti);
> > +		preload_add(ti, pc);
> > +	}
> > +
> > +	for (i = 0; i < ti->slb_preload_nr; i++) {
> > +		unsigned char idx;
> > +		unsigned long ea;
> > +
> > +		idx = (ti->slb_preload_tail + i) % SLB_PRELOAD_NR;
> > +		ea = (unsigned long)ti->slb_preload_esid[idx] << SID_SHIFT;
> > +
> > +		slb_allocate_user(mm, ea);
> > +	}
> > +}
> > +
> >  /* Flush all user entries from the segment table of the current processor. */
> >  void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
> >  {
> > -	struct thread_info *ti = task_thread_info(tsk);
> >  	unsigned char i;
> >  
> >  	/*
> > @@ -502,29 +531,8 @@ void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
> >  
> >  	copy_mm_to_paca(mm);
> >  
> > -	/*
> > -	 * We gradually age out SLBs after a number of context switches to
> > -	 * reduce reload overhead of unused entries (like we do with FP/VEC
> > -	 * reload). Each time we wrap 256 switches, take an entry out of the
> > -	 * SLB preload cache.
> > -	 */
> > -	tsk->thread.load_slb++;
> > -	if (!tsk->thread.load_slb) {
> > -		unsigned long pc = KSTK_EIP(tsk);
> > -
> > -		preload_age(ti);
> > -		preload_add(ti, pc);
> > -	}
> > -
> > -	for (i = 0; i < ti->slb_preload_nr; i++) {
> > -		unsigned char idx;
> > -		unsigned long ea;
> > -
> > -		idx = (ti->slb_preload_tail + i) % SLB_PRELOAD_NR;
> > -		ea = (unsigned long)ti->slb_preload_esid[idx] << SID_SHIFT;
> > -
> > -		slb_allocate_user(mm, ea);
> > -	}
> > +	if (!mm->context.skip_slb_preload)
> > +		preload_slb_entries(tsk, mm);
>
> Should this be wrapped in likely()?

Seems like a good idea - yes.

>
> >  
> >  	/*
> >  	 * Synchronize slbmte preloads with possible subsequent user memory
>
> Right below this comment is the isync. It seems to be specifically
> concerned with synchronising preloaded slbs. Do you need it if you are
> skipping SLB preloads?
>
> It's probably not a big deal to have an extra isync in the fairly rare
> path when we're skipping preloads, but I thought I'd check.

I don't _think_ we need the `isync` if we are skipping the SLB preloads,
but then again it was always in the code-path before. If someone can
make a compelling argument to drop it when not preloading SLBs I will,
otherwise (considering some of the other non-obvious things I stepped
into with the Hash code) I will keep it here for now.

Thanks for the comments!

>
> Kind regards,
> Daniel
>
> > -- 
> > 2.26.1

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