Re: [PATCH mm-unstable v1 11/26] microblaze/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

David Hildenbrand
2023-02-27T17:02:41Z

>>>> /*
>>>> * Externally used page protection values.
>>>> diff --git a/arch/microblaze/include/asm/pgtable.h b/arch/microblaze/include/asm/pgtable.h
>>>> index 42f5988e998b..7e3de54bf426 100644
>>>> --- a/arch/microblaze/include/asm/pgtable.h
>>>> +++ b/arch/microblaze/include/asm/pgtable.h
>>>> @@ -131,10 +131,10 @@ extern pte_t *va_to_pte(unsigned long address);
>>>> * of the 16 available. Bit 24-26 of the TLB are cleared in the TLB
>>>> * miss handler. Bit 27 is PAGE_USER, thus selecting the correct
>>>> * zone.
>>>> - * - PRESENT *must* be in the bottom two bits because swap cache
>>>> - * entries use the top 30 bits. Because 4xx doesn't support SMP
>>>> - * anyway, M is irrelevant so we borrow it for PAGE_PRESENT. Bit 30
>>>> - * is cleared in the TLB miss handler before the TLB entry is loaded.
>>>> + * - PRESENT *must* be in the bottom two bits because swap PTEs use the top
>>>> + * 30 bits. Because 4xx doesn't support SMP anyway, M is irrelevant so we
>>>> + * borrow it for PAGE_PRESENT. Bit 30 is cleared in the TLB miss handler
>>>> + * before the TLB entry is loaded.
>>>
>>> So the PowerPC 4xx comment is still here?
>>
>> I only dropped the comment above __swp_type(). I guess you mean that we
>> could also drop the "Because 4xx doesn't support SMP anyway, M is
>> irrelevant so we borrow it for PAGE_PRESENT." sentence, correct? Not
>
> Yes, that's what I meant.
>
>> sure about the "Bit 30 is cleared in the TLB miss handler" comment, if
>> that can similarly be dropped.
>
> No idea, didn't check. But if it was copied from PPC, chances are
> high it's no longer true....

I'll have a look.

>
>>>> * - All other bits of the PTE are loaded into TLBLO without
>>>> * * modification, leaving us only the bits 20, 21, 24, 25, 26, 30 for
>>>> * software PTE bits. We actually use bits 21, 24, 25, and
>>>> @@ -155,6 +155,9 @@ extern pte_t *va_to_pte(unsigned long address);
>>>> #define _PAGE_ACCESSED 0x400 /* software: R: page referenced */
>>>> #define _PMD_PRESENT PAGE_MASK
>>>>
>>>> +/* We borrow bit 24 to store the exclusive marker in swap PTEs. */
>>>> +#define _PAGE_SWP_EXCLUSIVE _PAGE_DIRTY
>>>
>>> _PAGE_DIRTY is 0x80, so this is also bit 7, thus the new comment is
>>> wrong?
>>
>> In the example, I use MSB-0 bit numbering (which I determined to be
>> correct in microblaze context eventually, but I got confused a couple a
>> times because it's very inconsistent). That should be MSB-0 bit 24.
>
> Thanks, TIL microblaze uses IBM bit numbering...

I assume IBM bit numbering corresponds to MSB-0 bit numbering, correct?

I recall that I used the comment above "/* Definitions for MicroBlaze.
*/" as an orientation.

0 1 2 3 4 ... 18 19 20 21 22 23 24 25 26 27 28 29 30 31
RPN..................... 0 0 EX WR ZSEL....... W I M G

So ... either we adjust both or we leave it as is. (again, depends on
what the right thing to to is -- which I don't know :) )

--
Thanks,

David / dhildenb