Re: [PATCH, V3] parisc: Rewrite cache flush code for PA8800/PA8900

[Helge Deller <deller@gmx.de>]

On 5/17/22 15:19, Rolf Eike Beer wrote:
> Am Montag, 16. Mai 2022, 23:49:10 CEST schrieb Helge Deller:
>> On 5/16/22 23:28, Rolf Eike Beer wrote:
>>> Am Montag, 16. Mai 2022, 17:14:47 CEST schrieb John David Anglin:
>>>> Originally, I was convinced that we needed to use tmpalias flushes
>>>> everwhere, for both user and kernel flushes. However, when I modified
>>>> flush_kernel_dcache_page_addr, to use a tmpalias flush, my c8000
>>>> would crash quite early when booting.
>>>>
>>>> The PDC returns alias values of 0 for the icache and dcache. This
>>>> indicates that either the alias boundary is greater than 16MB or
>>>> equivalent aliasing doesn't work. I modified the tmpalias code to
>>>> make it easy to try alternate boundaries. I tried boundaries up to
>>>> 128MB but still kernel tmpalias flushes didn't work on c8000.
>>>>
>>>> This led me to conclude that tmpalias flushes don't work on PA8800
>>>> and PA8900 machines, and that we needed to flush directly using the
>>>> virtual address of user and kernel pages. This is likely the major
>>>> cause of instability on the c8000 and rp34xx machines.
>>>>
>>>> Flushing user pages requires doing a temporary context switch as we
>>>> have to flush pages that don't belong to the current context. Further,
>>>> we have to deal with pages that aren't present. If a page isn't
>>>> present, the flush instructions fault on every line.
>>>>
>>>> Other code has been rearranged and simplified based on testing. For
>>>> example, I introduced a flush_cache_dup_mm routine. flush_cache_mm
>>>> and flush_cache_dup_mm differ in that flush_cache_mm calls
>>>> purge_cache_pages and flush_cache_dup_mm calls flush_cache_pages.
>>>> In some implementations, pdc is more efficient than fdc. Based on
>>>> my testing, I don't believe there's any performance benefit on the
>>>> c8000.
>>>>
>>>> V2:
>>>> 1) Add flush_cache_page_check_pte routine.
>>>> 2) Use it in copy_to_user_page and copy_from_user_page.
>>>> 3) flush_anon_page moved to cache.c and updated.
>>>> 4) Changed alias boundary to 64 MB for 64-bit kernels. Revised comment
>>>>
>>>>    regarding alias boundary for PA8800/PA8900 processors.
>>>>
>>>> 5) Removed struct mm_struct * argument from flush_cache_pages.
>>>> 6) Fixed thinko in flush_cache_range. It increased the number of pages
>>>>
>>>>    flushed and slowed performance.
>>>>
>>>> 7) Removed sync changes from pacache.S.
>>>>
>>>> V3:
>>>> 1) copy_to_user_page and copy_from_user_page moved to cache.c to
>>>>
>>>>    improve inlining.
>>>>
>>>> 2) Replaced copy_user_page with copy_user_highpage.
>>>> 3) Fixed cache threshold calculation on 32-bit kernels.
>>>> 4) Don't warn on inequivalent private mappings in flush_dcache_page.
>>>> 5) Return early from mm_total_size if size exceeds
>>>>
>>>>    parisc_cache_flush_threshold.
>>>>
>>>> 6) Disable non-access TLB warning in handle_nadtlb_fault. Warning
>>>>
>>>>    happens occassionally handling flushes for COW faults.
>>>>
>>>> 7) Remove flush_cache_dup_mm.
>>>> 8) Flush entire cache in flush_cache_mm and flush_cache_range on
>>>>
>>>>    processors with aliasing caches. Only flush small cache ranges
>>>>    on machines with PA8800/PA8900 processors.
>>>>
>>>> 9) Tested on rp3440, c8000 and c3750.
>>>
>>> Given how long these changelogs are, and how fragile the whole caching is
>>> I
>>> think it is a good idea to split this patch into smaller ones, to improve
>>> readability and being able to bisect it.
>>
>> FWIW, I've done some cleanups to this patch and committed it to my for-next
>> tree. In case it's split up, please use the revised version.
>
> Why did you modify get_ptep()? Until now it was just moved around in the file,
> and IMHO it becomes less readable because all these needless variables are
> batched up at the start of the function now. The only point I would see in
> moving them all to the front is if there would be no nesting anymore, and
> every condition was inverted:

Dave's original patch did not moved the variables to the beginning.
That change was me - just because checkpatch complained otherwise.

I agree that it's less readable.

Helge