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

Ryan Roberts
2023-12-12T11:47:12Z

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@arm.com>
>> ---
>> 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()?

>
> Will