linux-kernel - Re: [PATCH 3/6] riscv: convert bottom half of exception handling to C

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Date: Wed, 19 Jun 2024 10:04:05 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Samuel Holland <samuel.holland@...ive.com>,
	Clement Leger <cleger@...osinc.com>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] riscv: convert bottom half of exception handling to C

On Mon, Jun 17, 2024 at 01:05:50AM +0800, Jisheng Zhang wrote:
>For readability, maintainability and future scalability, convert the
>bottom half of the exception handling to C.
>
>Mostly the assembly code is converted to C in a relatively
>straightforward manner.
>
>However, there are two modifications I need to mention:
>
>1. the CSR_CAUSE reg reading and saving is moved to the C code
>because we need the cause to dispatch the exception handling,
>if we keep the cause reading and saving, we either pass it to
>do_traps() via. 2nd param or get it from pt_regs which an extra
>memory load is needed, I don't like any of the two solutions becase
>the exception handling sits in hot code path, every instruction
>matters.

CC: Clement.

I think its better to save away cause in pt_regs prior to calling
`do_traps`. Once control is transferred to C code in `do_traps`,
another trap can happen. It's a problem anyways today without CPU support.

Although with Ssdbltrp [1] extension and it kernel support [2] for it,
I expect asm code would clear up `SDT` bit in mstatus. Whenever `Ssdbltrp` lands,
I think `do_traps` should expect nesting of traps and thus cause should be saved
away before it gets control so that safely traps can be nested.

[1] - https://github.com/riscv/riscv-double-trap/releases/download/v1.0-rc1/riscv-double-trap.pdf
[2] - https://lore.kernel.org/all/20240418133916.1442471-1-cleger@rivosinc.com/

>
>2.To cope with SIFIVE_CIP_453 errata, it looks like we don't need
>alternative mechanism any more after the asm->c convertion. Just
>replace the excp_vect_table two entries.
>
>Signed-off-by: Jisheng Zhang <jszhang@...nel.org>