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

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Date: Thu, 20 Jun 2024 10:02:27 +1000
From: Cyril Bur <cyrilbur@...storrent.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: Jisheng Zhang <jszhang@...nel.org>, 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 Thu, Jun 20, 2024 at 3:04 AM Deepak Gupta <debug@...osinc.com> wrote:
>
> 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.
>

Is a possible solution to do both options Jisheng suggested? Save the
cause before
calling do_traps but also pass it via second param?

> [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>
>
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> linux-riscv@...ts.infradead.org
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