On 2022/11/14 11:29, Anshuman Khandual wrote:
>
>
> On 10/28/22 13:42, Yicong Yang wrote:
>> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
>> +{
>> + /*
>> + * TLB batched flush is proved to be beneficial for systems with large
>> + * number of CPUs, especially system with more than 8 CPUs. TLB shutdown
>> + * is cheap on small systems which may not need this feature. So use
>> + * a threshold for enabling this to avoid potential side effects on
>> + * these platforms.
>> + */
>> + if (num_online_cpus() <= CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
>> + return false;
>> +
>> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
>> + if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
>> + return false;
>> +#endif
>
> should_defer_flush() is immediately followed by set_tlb_ubc_flush_pending() which calls
> arch_tlbbatch_add_mm(), triggering the actual TLBI flush via __flush_tlb_page_nosync().
> It should be okay to check capability with this_cpu_has_cap() as the entire call chain
> here is executed on the same cpu. But just wondering if cpus_have_const_cap() would be
> simpler, consistent, and also cost effective ?
>
ok. Checked cpus_have_const_cap() I think it matches your words.
> Regardless, a comment is needed before the #ifdef block explaining why it does not make
> sense to defer/batch when __tlbi()/__tlbi_user() implementation will execute 'dsb(ish)'
> between two TLBI instructions to workaround the errata.
>
The workaround for the errata mentioned the affected platforms need the tlbi+dsb to be done
twice, so I'm not sure if we defer the final dsb will cause any problem so I think the judgement
here is used for safety. I have no such platform to test if it's ok to defer the last dsb.
>> +
>> + return true;
>> +}
>> +
>> +static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
>> + struct mm_struct *mm,
>> + unsigned long uaddr)
>> +{
>> + __flush_tlb_page_nosync(mm, uaddr);
>> +}
>> +
>> +static inline void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
>> +{
>> + dsb(ish);
>> +}
> .
>
