Re: io_uring failure on parisc with VIPT caches

[John David Anglin <dave.anglin@bell.net>]

On 2023-02-14 6:29 p.m., Jens Axboe wrote:
> On 2/14/23 4:09 PM, Helge Deller wrote:
>> * John David Anglin<dave.anglin@bell.net>:
>>> On 2023-02-13 5:05 p.m., Helge Deller wrote:
>>>> On 2/13/23 22:05, Jens Axboe wrote:
>>>>> On 2/13/23 1:59?PM, Helge Deller wrote:
>>>>>>> Yep sounds like it. What's the caching architecture of parisc?
>>>>>> parisc is Virtually Indexed, Physically Tagged (VIPT).
>>>>> That's what I assumed, so virtual aliasing is what we're dealing with
>>>>> here.
>>>>>
>>>>>> Thanks for the patch!
>>>>>> Sadly it doesn't fix the problem, as the kernel still sees
>>>>>> ctx->rings->sq.tail as being 0.
>>>>>> Interestingly it worked once (not reproduceable) directly after bootup,
>>>>>> which indicates that we at least look at the right address from kernel side.
>>>>>>
>>>>>> So, still needs more debugging/testing.
>>>>> It's not like this is untested stuff, so yeah it'll generally be
>>>>> correct, it just seems that parisc is a bit odd in that the virtual
>>>>> aliasing occurs between the kernel and userspace addresses too. At least
>>>>> that's what it seems like.
>>>> True.
>>>>
>>>>> But I wonder if what needs flushing is the user side, not the kernel
>>>>> side? Either that, or my patch is not flushing the right thing on the
>>>>> kernel side.
>> The patch below seems to fix the issue.
>>
>> I've successfuly tested it with the io_uring-test testcase on
>> physical parisc machines with 32- and 64-bit 6.1.11 kernels.
>>
>> The idea is similiar on how a file is mmapped shared by two
>> userspace processes by keeping the lower bits of the virtual address
>> the same.
>>
>> Cache flushes from userspace don't seem to be needed.
> Are they from the kernel side, if the lower bits mean we end up
> with the same coloring? Because I think this is a bit of a big
> hammer, in terms of overhead for flushing. As an example, on arm64
> that is perfectly fine with the existing code, it's about a 20-25%
> performance hit.
The io_uring-test testcase still works on rp3440 with the kernel flushes removed.

-- 
John David Anglin  dave.anglin@bell.net