RFC: extern illegal instruction trap and trap RDCYCLE

[Atish Patra]

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

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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