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

From: "Clément Léger" <cleger@rivosinc.com>
To: Cyril Bur <cyrilbur@tenstorrent.com>, Deepak Gupta <debug@rivosinc.com>
Cc: Jisheng Zhang <jszhang@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Samuel Holland <samuel.holland@sifive.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/6] riscv: convert bottom half of exception handling to C
Date: Thu, 20 Jun 2024 10:06:15 +0200	[thread overview]
Message-ID: <95a081a9-2307-4e5b-8ae2-082be16b327e@rivosinc.com> (raw)
In-Reply-To: <CANtDSipXbYSuz6NakbXw-8k0_ZN5hdT7VWMushXvQ5VB0Am_Ng@mail.gmail.com>

On 20/06/2024 02:02, Cyril Bur wrote:
> On Thu, Jun 20, 2024 at 3:04 AM Deepak Gupta <debug@rivosinc.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.

Hi,

Indeed, every register that is "unique" to a trap and than can be
overwritten by a second trap should be saved before reenabling them when
using Ssdbltrp. So that would be nice to preserve that.

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

I guess so if it fits your performance requirements.

Thanks,

Clément

> 
>> [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@kernel.org>
>>
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