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Date: Thu, 01 Jul 2021 14:15:38 +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 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?
Thanks,
Nick
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