Message-ID: <6e2ed6a9-aed0-4727-b1e3-903d3a7751b5@rivosinc.com>
Date: Tue, 22 Apr 2025 12:17:41 +0200
From: Clément Léger <cleger@...osinc.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Alexandre Ghiti <alex@...ti.fr>,
 "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>,
 "open list:RISC-V ARCHITECTURE" <linux-riscv@...ts.infradead.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
 Jonathan Corbet <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Shuah Khan <shuah@...nel.org>, Andrew Jones <ajones@...tanamicro.com>,
 Samuel Holland <samuel.holland@...ive.com>
Subject: Re: [PATCH 1/5] riscv: misaligned: factorize trap handling



On 22/04/2025 11:44, Peter Zijlstra wrote:
> On Tue, Apr 22, 2025 at 09:57:12AM +0200, Clément Léger wrote:
>>
>>
>> On 21/04/2025 09:06, Alexandre Ghiti wrote:
>>> Hi Clément,
>>>
>>>
>>> On 14/04/2025 14:34, Clément Léger wrote:
>>>> misaligned accesses traps are not nmi and should be treated as normal
>>>> one using irqentry_enter()/exit().
>>>
>>>
>>> All the traps that come from kernel mode are treated as nmi as it was
>>> suggested by Peter here: https://lore.kernel.org/linux-riscv/
>>> Yyhv4UUXuSfvMOw+@...ez.programming.kicks-ass.net/
>>>
>>> I don't know the differences between irq_nmi_entry/exit() and irq_entry/
>>> exit(), so is that still correct to now treat the kernel traps as non-nmi?
>>
>> Hi Alex,
>>
>> Actually, this discussion was raised on a previous series [1] by Maciej
>> which replied that we should actually reenable interrupt depending on
>> the state that was interrupted. Looking at other architecture/code, it
>> seems like treating misaligned accesses as NMI is probably not the right
>> way. For instance, loongarch treats them as normal IRQ using a
>> irqentry_enter()/exit() and reenabling IRQS if possible.
> 
> So, a trap that happens in kernel space while IRQs are disabled, SHOULD
> really be NMI-like.
> 
> You then have a choice, make all such traps from kernel space NMI-like;
> this makes it easy on the trap handler, since the context is always the
> same. Mistakes are 'easy' to find.
> 
> Or,.. do funny stuff and only make it NMI like if IRQs were disabled.
> Which gives inconsistent context for the handler and you'll find
> yourself scratching your head at some point in the future wondering why
> this one rare occasion goes BOOM.

Hi Peter,

Yeah agreed, that might be hazardous.

> 
> x86 mostly does the first, any trap that can happen with IRQs disabled
> is treated unconditionally as NMI like. The obvious exception is
> page-fault, but that already has a from-non-preemptible-context branch
> that is 'careful'.
> 
> As to unaligned traps from kernel space, I would imagine they mostly BUG
> the kernel, except when there's an exception entry for that location, in
> which case it might do a fixup?

The misaligned access exception handling currently handles misaligned
access for the kernel as well (except if explicitly disabled).

> 
> 
> Anyway, the reason these exceptions should be NMI like, is because
> interrupts are not allowed to nest. Notably something like:
> 
> 
>   raw_spin_lock_irqsave(&foo);
>   <IRQ>
>     raw_spin_lock_irqsave(&foo);
>     ...
> 
> Is an obvious problem. Exceptions that can run while IRQs are disabled,
> must not use locks -- treating them as NMI-like (they are non-maskable
> after all), ensures this.

As said in my previous reply, the misaligned handling code does not
currently access any locks (when handling kernel misaligned accesses)
and is self contained. That being said, that assumption might not be
true in future so that might be better to take your approach and treat
the misaligned access like an NMI.

Thanks,

Clément

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