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Date:   Thu, 01 Jul 2021 17:37:16 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     "Christopher M. Riedl" <cmr@...ux.ibm.com>,
        Daniel Axtens <dja@...ens.net>, linuxppc-dev@...ts.ozlabs.org
Cc:     keescook@...omium.org, linux-hardening@...r.kernel.org,
        tglx@...utronix.de, x86@...nel.org
Subject: Re: [RESEND PATCH v4 05/11] powerpc/64s: Add ability to skip SLB
 preload

Excerpts from Christopher M. Riedl's message of July 1, 2021 4:53 pm:
> On Thu Jul 1, 2021 at 1:04 AM CDT, Nicholas Piggin wrote:
>> Excerpts from Christopher M. Riedl's message of July 1, 2021 3:28 pm:
>> > On Wed Jun 30, 2021 at 11:15 PM CDT, Nicholas Piggin wrote:
>> >> Excerpts from Christopher M. Riedl's message of July 1, 2021 1:48 pm:
>> >> > 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.
>> >>
>> >> The ISA says slbia wants an isync afterward, so we probably should keep
>> >> it. The comment is a bit misleading in that case.
>> >>
>> >> Why isn't preloading appropriate for a temporary mm?
>> > 
>> > The preloaded entries come from the thread_info struct which isn't
>> > necessarily related to the temporary mm at all. I saw SLB multihits
>> > while testing this series with my LKDTM test where the "patching
>> > address" (userspace address for the temporary mapping w/
>> > write-permissions) ends up in a thread's preload list and then we
>> > explicitly insert it again in map_patch() when trying to patch. At that
>> > point the SLB multihit triggers.
>>
>> Hmm, so what if we use a mm, take some SLB faults then unuse it and
>> use a different one? I wonder if kthread_use_mm has existing problems
>> with this incorrect SLB preloading. Quite possibly. We should clear
>> the preload whenever mm changes I think. That should cover this as
>> well.
> 
> I actually did this initially but thought it was a bit too intrusive to
> include as part of this series and hurt performance. I agree that
> preloading the SLB from the thread may be a problem in general when
> switching in/out an mm.
> 
> kthread_use_mm may not be affected unless we explicitly insert some SLB
> entries which could collide with an existing preload (which I don't
> think we do anywhere until this series).

kthread_use_mm(mm1);
*ea = blah; /* slb preload[n++][ea] = va */
kthread_unuse_mm(mm1);

kthread_use_mm(mm2);
  switch_slb();
schedule();
  /* preload ea=va? */
x = *ea;
kthread_unuse_mm(mm2);

? I'm sure we'd have a bug in existing code if you're hitting a bug 
there.

Thanks,
Nick

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