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

[Ryan Roberts]

Hi Will,

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:
>>> Split __flush_tlb_range() into __flush_tlb_range_nosync() +
>>> __flush_tlb_range(), in the same way as the existing flush_tlb_page()
>>> arrangement. This allows calling __flush_tlb_range_nosync() to elide the
>>> trailing DSB. Forthcoming "contpte" code will take advantage of this
>>> when clearing the young bit from a contiguous range of ptes.
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>> ---
>>>  arch/arm64/include/asm/tlbflush.h | 13 +++++++++++--
>>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> 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?

Thanks,
Ryan


> 
> 
> 
>>
>> Will
>