Re: [PATCH v3 12/15] arm64/mm: Split __flush_tlb_range() to elide trailing DSB

[Ryan Roberts]

On 14/12/2023 12:30, Robin Murphy wrote:
> On 2023-12-14 12:13 pm, Will Deacon wrote:
>> On Thu, Dec 14, 2023 at 11:53:52AM +0000, Ryan Roberts wrote:
>>> On 12/12/2023 11:47, Ryan Roberts wrote:
>>>> On 12/12/2023 11:35, Will Deacon wrote:
>>>>> On Mon, Dec 04, 2023 at 10:54:37AM +0000, Ryan Roberts wrote:
>>>>>> diff --git a/arch/arm64/include/asm/tlbflush.h
>>>>>> b/arch/arm64/include/asm/tlbflush.h
>>>>>> index bb2c2833a987..925ef3bdf9ed 100644
>>>>>> --- a/arch/arm64/include/asm/tlbflush.h
>>>>>> +++ b/arch/arm64/include/asm/tlbflush.h
>>>>>> @@ -399,7 +399,7 @@ do {                                    \
>>>>>>   #define __flush_s2_tlb_range_op(op, start, pages, stride, tlb_level) \
>>>>>>       __flush_tlb_range_op(op, start, pages, stride, 0, tlb_level, false)
>>>>>>   -static inline void __flush_tlb_range(struct vm_area_struct *vma,
>>>>>> +static inline void __flush_tlb_range_nosync(struct vm_area_struct *vma,
>>>>>>                        unsigned long start, unsigned long end,
>>>>>>                        unsigned long stride, bool last_level,
>>>>>>                        int tlb_level)
>>>>>> @@ -431,10 +431,19 @@ static inline void __flush_tlb_range(struct
>>>>>> vm_area_struct *vma,
>>>>>>       else
>>>>>>           __flush_tlb_range_op(vae1is, start, pages, stride, asid,
>>>>>> tlb_level, true);
>>>>>>   -    dsb(ish);
>>>>>>       mmu_notifier_arch_invalidate_secondary_tlbs(vma->vm_mm, start, end);
>>>>>>   }
>>>>>>   +static inline void __flush_tlb_range(struct vm_area_struct *vma,
>>>>>> +                     unsigned long start, unsigned long end,
>>>>>> +                     unsigned long stride, bool last_level,
>>>>>> +                     int tlb_level)
>>>>>> +{
>>>>>> +    __flush_tlb_range_nosync(vma, start, end, stride,
>>>>>> +                 last_level, tlb_level);
>>>>>> +    dsb(ish);
>>>>>> +}
>>>>>
>>>>> Hmm, are you sure it's safe to defer the DSB until after the secondary TLB
>>>>> invalidation? It will have a subtle effect on e.g. an SMMU participating
>>>>> in broadcast TLB maintenance, because now the ATC will be invalidated
>>>>> before completion of the TLB invalidation and it's not obviously safe to me.
>>>>
>>>> I'll be honest; I don't know that it's safe. The notifier calls turned up
>>>> during
>>>> a rebase and I stared at it for a while, before eventually concluding that I
>>>> should just follow the existing pattern in __flush_tlb_page_nosync(): That one
>>>> calls the mmu notifier without the dsb, then flush_tlb_page() does the dsb
>>>> after. So I assumed it was safe.
>>>>
>>>> If you think it's not safe, I guess there is a bug to fix in
>>>> __flush_tlb_page_nosync()?
>>>
>>> Did you have an opinion on this? I'm just putting together a v4 of this series,
>>> and I'll remove this optimization if you think it's unsound. But in that case, I
>>> guess we have an existing bug to fix too?
>>
>> Sorry, Ryan, I've not had a chance to look into it in more detail. But as
>> you rightly point out, you're not introducing the issue (assuming it is
>> one), so I don't think it needs to hold you up. Your code just makes the
>> thing more "obvious" to me.

OK thanks. I'll leave my code as is for now then - that makes it easier to do
A/B performance comparison with the existing code. And I can change it if/when
mainline changes (presumably to add the dsb between the tlbi and the mmu
notifier callback).

>>
>> Robin, Jean-Philippe -- do we need to make sure that the SMMU has completed
>> its TLB invalidation before issuing an ATC invalidate? My half-baked worry
>> is whether or not an ATS request could refill the ATC before the TLBI
>> has completed, therefore rendering the ATC invalidation useless.
> 
> I would agree, and the spec for CMD_ATC_INV does call out a
> TLBI->sync->ATCI->sync sequence. At the moment the SVA notifier is issuing its
> own command-based TLBIs anyway so the necessary sync is implicit there, but if
> and when we get BTM support wired up properly it would be nice not to have to
> bodge in an additional sync/DSB.
> 
> Cheers,
> Robin.