Re: [PATCH] riscv: Fix 32-bit call_on_irq_stack() frame pointer ABI

From: Nam Cao <namcao@linutronix.de>
To: Michael Ellerman <mpe@kernel.org>,
	Rui Qi <qirui.001@bytedance.com>,
	palmer@dabbelt.com, pjw@kernel.org, aou@eecs.berkeley.edu,
	alex@ghiti.fr, conor@kernel.org
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	samuel.holland@sifive.com
Subject: Re: [PATCH] riscv: Fix 32-bit call_on_irq_stack() frame pointer ABI
Date: Thu, 25 Jun 2026 10:49:05 +0200	[thread overview]
Message-ID: <87pl1fvwce.fsf@yellow.woof> (raw)
In-Reply-To: <2e47388a-8883-4b44-b440-a00b8e622733@kernel.org>

Michael Ellerman <mpe@kernel.org> writes:
> On 24/6/26 9:31 pm, Rui Qi wrote:
>> call_on_irq_stack() uses struct member offsets to set up its link in the
>> frame record list. On riscv32, struct stackframe is the wrong size to
>> maintain stack pointer alignment, so STACKFRAME_SIZE_ON_STACK includes
>> padding. However, the ABI requires the frame record to be placed
>> immediately below the address stored in s0, so the padding must come
>> before the struct members.
>> 
>> Fix the layout by making STACKFRAME_FP and STACKFRAME_RA the negative
>> offsets from s0, instead of the positive offsets from sp.
>
> The fact that all uses of the defines need to add back STACKFRAME_SIZE_ON_STACK
> makes me think the defines don't have the most useful values.
>
> If the values were offset + padding then the uses could be left unchanged.

At the moment we only have only one user which has nothing except the
stack frame on the stack. Other potential users do not necessarily add back
STACKFRAME_SIZE_ON_STACK.

The offset value depends on the function. So I think it's better to
leave that offset calculation to users.

Nam

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