Re: RFC: extern illegal instruction trap and trap RDCYCLE

Ben Dooks
2024-09-18T09:59:29Z

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@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

>
> 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
>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> linux-riscv mailing list
>>>> linux-riscv@lists.infradead.org
>>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>>
>>> _______________________________________________
>>> linux-riscv mailing list
>>> linux-riscv@lists.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>>
>>
>>
>
>

--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius