Re: [PATCH v2] powerpc/traps: Enhance readability for trap types

Regards,
Xiongwei

On Apr 2, 2021, at 8:36 AM, Nicholas Piggin [off-list ref] wrote:

Excerpts from Segher Boessenkool's message of April 2, 2021 2:11 am:

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On Thu, Apr 01, 2021 at 10:55:58AM +0800, Xiongwei Song wrote:

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Segher Boessenkool [off-list ref] 于2021年4月1日周四 上午6:15写道：

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On Wed, Mar 31, 2021 at 08:58:17PM +1100, Michael Ellerman wrote:

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So perhaps:

 EXC_SYSTEM_RESET
 EXC_MACHINE_CHECK
 EXC_DATA_STORAGE
 EXC_DATA_SEGMENT
 EXC_INST_STORAGE
 EXC_INST_SEGMENT
 EXC_EXTERNAL_INTERRUPT
 EXC_ALIGNMENT
 EXC_PROGRAM_CHECK
 EXC_FP_UNAVAILABLE
 EXC_DECREMENTER
 EXC_HV_DECREMENTER
 EXC_SYSTEM_CALL
 EXC_HV_DATA_STORAGE
 EXC_PERF_MONITOR

These are interrupt (vectors), not exceptions.  It doesn't matter all
that much, but confusing things more isn't useful either!  There can be
multiple exceptions that all can trigger the same interrupt.

When looking at the reference manual of e500 and e600 from NXP

official, they call them as interrupts.While looking at the "The
Programming Environments"
that is also from NXP, they call them exceptions. Looks like there is
no explicit distinction between interrupts and exceptions.

The architecture documents have always called it interrupts.  The PEM
says it calls them exceptions instead, but they are called interrupts in
the architecture (and the PEM says that, too).

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Here is the "The Programming Environments" link:
https://www.nxp.com.cn/docs/en/user-guide/MPCFPE_AD_R1.pdf

That document is 24 years old.  The architecture is still published,
new versions regularly.

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As far as I know, the values of interrupts or exceptions above are defined
explicitly in reference manual or the programming environments.

They are defined in the architecture.

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Could
you please provide more details about multiple exceptions with the same
interrupts?

The simplest example is 700, program interrupt.  There are many causes
for it, including all the exceptions in FPSCR: VX, ZX, OX, UX, XX, and
VX is actually divided into nine separate cases itself.  There also are
the various causes of privileged instruction type program interrupts,
and  the trap type program interrupt, but the FEX ones are most obvious
here.

Also:

* Some interrupts have no corresponding exception (system call and 
system call vectored). This is not just semantics or a bug in the ISA
because it is different from other synchronous interrupts: instructions 
which cause exceptions (e.g., a page fault) do not complete before 
taking the interrupt whereas sc does.

* It's quite usual for an exception to not cause an interrupt 
immediately (MSR[EE]=0, HMEER) or never cause one and be cleared by 
other means (msgclr, mtDEC, mtHMER, etc).

* It's possible for an exception to cause different interrupts!
A decrementer exception usually causes a decrementer interrupt, but it
can cause a system reset interrupt if the processor was in a power
saving mode. A data storage exception can cause a DSI or HDSI interrupt
depending on LPCR settings, and many other examples.

So I agree with Segher on this. We should use interrupt for interrupts, 
reduce exception except where we really mean it, and move away from vec 
and trap (I've got this wrong in the past too I admit). We don't have to 
do it all immediately, but new code should go in this direction.

Appreciate these details about exceptions and interrupts. Looks like interrupt
is the correct term here. I’m glad to create the v3 patch with INTERRUPT_
prefix.  

Regards,
Xiongwei

Thanks,
Nick