RFC: extern illegal instruction trap and trap RDCYCLE

[Alexandre Ghiti]

Hi Atish, Ben,

On 19/09/2024 01:23, Atish Kumar Patra wrote:
> On Wed, Sep 18, 2024 at 2:57 AM Ben Dooks <ben.dooks at codethink.co.uk> wrote:
>> On 18/09/2024 10:55, Atish Patra wrote:
>>> On 9/17/24 7:01 AM, Ben Dooks wrote:
>>>> On 17/09/2024 14:46, Palmer Dabbelt wrote:
>>>>> On Tue, 17 Sep 2024 06:08:50 PDT (-0700), ben.dooks at codethink.co.uk
>>>>> wrote:
>>>>>> This is a RFC series to change how the illegal instruction trap
>>>>>> is handled and then how to trap RDCYCLE and emulate it with RDTIME
>>>>>> instead.
>>>>> Only 1/3 made it to lore for me, not sure if it's just stuck somewhere.
>>>>>
>>>>>> I did this when we found multiple libraries using RDCYCLE and
>>>>>> upgrading multiple runners to newer kernels caused many problems
>>>>> OK, so I think we're kind of just stuck with RDCYCLE then -- it was
>>>>> part of the base ISA when we merged the port, and every time it
>>>>> disappears we end up breaking userspace.
>>>> Yes, it was an annoyance, we managed to track down all the users and
>>>> patch out.
>>>>
>>>> I thought this might be a useful idea for more generic type, so might
>>>> look at updating to have a table of instruction masks to call.
>>>>
>>>>> I'm not sure what exactly the right way to do this is: IIRC there's
>>>>> some perf-related hooks for this, but there's also systems that just
>>>>> don't implement the RDCYCLE instruction at all and thus we'll need
>>>>> some sort of emulation for those.
>>>> I couldn't get the PMU driver to allow it, not sure if there was an
>>>> issue higher up or some other issue?
>>>>
>>> Here is the way to properly allow it via the driver.
>>>
>>> https://www.kernel.org/doc/Documentation/admin-guide/sysctl/kernel.rst
>>>
>>> Check the perf_user_access
>> Didn't work for me, not sure why
>>
> +Alexandre Ghiti
>
> Hmm. That's weird. Maybe some bug crept in recently. Can you provide
> some more details
> on the platform, kernel version and procedure followed to change it ?


So I have just tried perf_user_access on 6.11 and it works fine, meaning 
setting the "legacy" behaviour (ie 2) allows a userspace application to 
directly access RDCYCLE.

Can you provide more details Ben?

Thanks,

Alex


>
>>> As you noted, the user space application shouldn't use RDCYCLE for
>>> various reasons discussed in the past.
>>>
>>> If RDTIME is really not an option, you can always set the legacy mode to
>>> enable access.
>>>
>>>>> So hopefully the other two patches get through the lists at some
>>>>> point, but I think in general this is a reasonable thing to do -- or
>>>>> I guess maybe a completely unreasonable thing to be stuck needing to
>>>>> do, but no way around it ;)
>>>> I'll check back later, currently as OSS-EU
>>>>
>>>>>>
>>>>>>
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>>
>> --
>> Ben Dooks                               http://www.codethink.co.uk/
>> Senior Engineer                         Codethink - Providing Genius
>>
>> https://www.codethink.co.uk/privacy.html
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