linux-kernel - Re: [PATCH 02/11] riscv: update asm call site in `call_on_irq

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Message-ID: <20250725153351.GB1724026@google.com>
Date: Fri, 25 Jul 2025 15:33:51 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
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	Michal Hocko <mhocko@...e.com>,
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	Monk Chiang <monk.chiang@...ive.com>,
	Kito Cheng <kito.cheng@...ive.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
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	ved@...osinc.com
Subject: Re: [PATCH 02/11] riscv: update asm call site in `call_on_irq_stack`
 to setup correct label

On Thu, Jul 24, 2025 at 04:36:55PM -0700, Deepak Gupta wrote:
> Call sites written in asm performing indirect call, they need to setup
> label register (t2/x7) with correct label.
> 
> Currently first kernel was compiled with `-save-temps` option and
> normalized function signature string is captured and then placed at the
> asm callsite.
> 
> TODO: to write a macro wrapper with toolchain support.
> 
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>  arch/riscv/kernel/entry.S | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 2660faf52232..598e17e800ae 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -389,6 +389,7 @@ SYM_FUNC_START(call_on_irq_stack)
>  	load_per_cpu t0, irq_stack_ptr, t1
>  	li	t1, IRQ_STACK_SIZE
>  	add	sp, t0, t1
> +	lui t2, %lpad_hash("FvP7pt_regsE")

Ah, I see. The plan is to hardcode the signatures in assembly code and
keep them manually in sync with C code. When we implemented KCFI, we
thought this would become extremely tedious to maintain after a while.
Do you have any plans to add KCFI-style __kcfi_typeid_<function> symbols
that would allow labels to be determined from C type signatures instead?

Sami