[PATCH] Documentation: Fill the gaps about entry/noinstr constraints

[Thomas Gleixner]

Mark,

On Wed, Dec 01 2021 at 10:56, Mark Rutland wrote:
> On Tue, Nov 30, 2021 at 11:31:30PM +0100, Thomas Gleixner wrote:
>> ---
>>  Documentation/core-api/entry.rst |  268 +++++++++++++++++++++++++++++++++++++++
>>  Documentation/core-api/index.rst |    8 +
>>  kernel/entry/common.c            |    1 
>
> I think the change to kernel/entry/common.c got included by accident?

That's what I get from doing such things 30 minutes before midnight...

>> +
>> +Syscall entry exit code starts obviously in low level architecture specific
>
> As a small nit, can we remove the "obviously"? It's certainly obvious to you
> and me, but it doesn't meaningfully affect the sentence either way.

Indeed.

>> +assembly code and calls out into C-code after establishing low level
>> +architecture specific state and stack frames. This low level code must not
>> +be instrumented. A typical syscall handling function invoked from low level
>> +assembly code looks like this::
>> +
>> +  noinstr void do_syscall(struct pt_regs \*regs, int nr)
>                                             ^^
>
> Is `\*` necessary here? ... and/or should this be an explicit code block (which
> IIUC doesn't require this esacping), e.g.
>
> .. code-block:: c

Right. Let me try that.

>       noinstr void do_syscall(struct pt_regs *regs, int nr)
>> +
>> +If the interrupt is raised while the CPU executes in kernel space the entry
>> +and exit handling is slightly different. RCU state is only updated when the
>> +interrupt was raised in context of the idle task because that's the only
>
> Since we have an idle task for each cpu, perhaps either:
>
>   s/the idle task/an idle task/
>   s/the idle task/the CPU's idle task/

Yes, that's more precise

>> +Note, that the update of the preemption counter has to be the first
>> +operation on enter and the last operation on exit. The reason is that both
>> +lockdep and RCU rely on in_nmi() returning true in this case. The
>> +preemption count modification in the NMI entry/exit case can obviously not
>> +be traced.
>
> Could we say "must not" instead of "can not", e.g.
>
>   The preemption count modification in the NMI entry/exit must not be traced.
>
> That way it's clearly a requirement, rather than a limitation.

Yes.

>> +Architecture specific code looks like this::
>> +
>> +  noinstr void do_nmi(struct pt_regs \*regs)
>> +  {
>> +	arch_nmi_enter(regs);
>> +	state = irqentry_nmi_enter(regs);
>> +
>> +	instrumentation_begin();
>> +
>> +	invoke_nmi_handler(regs);
>> +
>> +	instrumentation_end();
>> +	irqentry_nmi_exit(regs);
>> +  }
>
> To keep the begin/end and enter/exit calls visually balanced, should the
> instrumentation_end() call have trailing a line space, e.g.

Yup.

Thanks,

        tglx