Re: [PATCH v1] s390x/tcg: Implement MONITOR CALL

[David Hildenbrand]

On 17.09.20 17:33, Richard Henderson wrote:
> On 9/17/20 4:54 AM, David Hildenbrand wrote:
>> Recent upstream Linux uses the MONITOR CALL instruction for things like
>> BUG_ON() and WARN_ON(). We currently inject an operation exception when
>> we hit a MONITOR CALL instruction - which is wrong, as the instruction
>> is not glued to specific CPU features.
>>
>> Doing a simple WARN_ON_ONCE() currently results in a panic:
>>   [   18.162801] illegal operation: 0001 ilc:2 [#1] SMP
>>   [   18.162889] Modules linked in:
>>   [...]
>>   [   18.165476] Kernel panic - not syncing: Fatal exception: panic_on_oops
>>
>> With a proper implementation, we now get:
>>   [   18.242754] ------------[ cut here ]------------
>>   [   18.242855] WARNING: CPU: 7 PID: 1 at init/main.c:1534 [...]
>>   [   18.242919] Modules linked in:
>>   [...]
>>   [   18.246262] ---[ end trace a420477d71dc97b4 ]---
>>   [   18.259014] Freeing unused kernel memory: 4220K
>>
>> To be able to translate it to a NOP easily, mangle the 16 monitor masks
>> bits from the cr8 into the TB flags.
> 
> This is a rare situation that does not warrant the use of TB flags.  Better to

I think it can be used for tracing and such, so quite excessively - but
yeah, at least in Linux it should be rare.

> unconditionally call helper_monitor_event, and have the helper function test
> the runtime value of cr8.  If the event is disabled, the helper simply 
> returns.
> 
> It should be simpler to write, as well, not having to do this:
> 

Was too appealing to optimize. Well, doing it in a handler also makes
the implementation of monitor-event counting easier.

-- 
Thanks,

David / dhildenb