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dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="tWUZpZhk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3F7A5C2BD10; Fri, 21 Jun 2024 00:10:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1718928655; bh=DVc+oZ8IpmajrWn8uMMOY6RL2NOLHfDxJNeCdHePKgg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tWUZpZhkar/bqgnu5QJIUR2WDZmrEDCqDQl/Wt9gCmNpuMJLA8RZ7aFzGwtt6L6lU X7/5/EvkILpOcBMSMR8MmL77N5k+UcTmddkUN998iHeaxqq/0byRHWYbIZ+XpYsN+c DEW/vRk9M0Z/yRExKwYEvfxTwndWbskG9LwaDC0zAqptY3Y+d2BInEYenQ2ul33HWF /un6QR868O/pDMl5JRnh0qbCtPygFX+30BM9qnZIRbat6ekFDNveQH7HK6tfgun0g6 DlpLrcuAbNwWctRWfbk0kvmyAib6nuI/G05ioQ84d6s9DGxnWIq2bHu8JrYWsu10tr ZYh4Ylb90JT7g== Date: Fri, 21 Jun 2024 07:56:56 +0800 From: Jisheng Zhang To: =?utf-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= Cc: Cyril Bur , Deepak Gupta , Paul Walmsley , Palmer Dabbelt , Albert Ou , Samuel Holland , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/6] riscv: convert bottom half of exception handling to C Message-ID: References: <20240616170553.2832-1-jszhang@kernel.org> <20240616170553.2832-4-jszhang@kernel.org> <95a081a9-2307-4e5b-8ae2-082be16b327e@rivosinc.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <95a081a9-2307-4e5b-8ae2-082be16b327e@rivosinc.com> On Thu, Jun 20, 2024 at 10:06:15AM +0200, Clément Léger wrote: > > > On 20/06/2024 02:02, Cyril Bur wrote: > > On Thu, Jun 20, 2024 at 3:04 AM Deepak Gupta 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, Hi Deepak, Clément, Currently, SR_IE bit is is set(setting means enable irq) in c, could the 'SDT' bit be cleared in c as well when Ssdbltrp lands? Thanks > >> 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 > >> > >> _______________________________________________ > >> linux-riscv mailing list > >> linux-riscv@lists.infradead.org > >> http://lists.infradead.org/mailman/listinfo/linux-riscv