Re: [patch 51/60] x86/mm: Allow flushing for future ASID switches

[Dave Hansen]

On 12/04/2017 02:22 PM, Andy Lutomirski wrote:
>> +
>> +       this_cpu_write(cpu_tlbstate.invalidate_other, true);
> 
> Why do we need this extra variable instead of just looping over all
> other ASIDs and invalidating them?  It would be something like:
> 
>         for (i = 1; i < TLB_NR_DYN_ASIDS; i++) {
>                 if (i != this_cpu_read(cpu_tlbstate.loaded_mm_asid))
>                        this_cpu_write(cpu_tlbstate.ctxs[i].ctx_id, 0);
>         }

We have loops like this:

	for (addr = start; addr < end; addr += PAGE_SIZE)
		flush_tlb_single();

Where flush_tlb_single() does a invalidate_pcid_other().  So, inlining
flush_tlb_single() rougly looks like:

	for (addr = start; addr < end; addr += PAGE_SIZE) {
		invlpg;
		for (i = 1; i < TLB_NR_DYN_ASIDS; i++) {
			this_cpu_write(cpu_tlbstate.ctxs[i].ctx_id, 0);
	}

or, with a "invalidate_other" variable:

	for (addr = start; addr < end; addr += PAGE_SIZE) {
		invlpg;
		this_cpu_write(cpu_tlbstate.ctxs.invalidate_other, 1);
	}

The double-for-loop looks a bit wasteful to me.


>>  static inline void __flush_tlb_one(unsigned long addr)
>>  {
>>         count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ONE);
>>         __flush_tlb_single(addr);
>> +       /*
>> +        * Invalidate other address spaces inaccessible to single-page
>> +        * invalidation:
>> +        */
> 
> Ugh.  If I'm reading this right, __flush_tlb_single() means "flush one
> user address" and __flush_tlb_one() means "flush one kernel address".
> That's, um, not exactly obvious.  Could this be at least commented
> better?

That sounds sane, but let me take a look at it.

Didn't Peter have some patches to do some of that rename?