From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9114DC63797 for ; Sun, 15 Jan 2023 16:29:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231322AbjAOQ3S (ORCPT ); Sun, 15 Jan 2023 11:29:18 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45394 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230358AbjAOQ3N (ORCPT ); Sun, 15 Jan 2023 11:29:13 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F21C24C13; Sun, 15 Jan 2023 08:29:11 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id AF6BCB80B32; Sun, 15 Jan 2023 16:29:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C4ECEC433D2; Sun, 15 Jan 2023 16:29:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1673800149; bh=Wt8nQy5JPCP9oRkQ5+sBiko1fzkFcMk22feZiNjlTU4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=hIshr8csUdqzGm8TkqeW8l06ROROLC2+1nMTcDzIuXx3x6Kl2wvCMjNlppAl2RNIc ozTVuTuPF50cWUHfS+E7u9koqaCZe96ws8Yjth0tUdzxzDwnkJM4tmJpanVOGcNacG A0VbWO6qgqVh96lzVTHX1f6eHu6WZlbEbQxzryl0huIBEh6HExUbZYm80rqAxhDKpg jzj39Zx4c3ReasolUUx5gTgttnu9itQ2L5ljgfOZdgDp8BS/iH8E6bN7vk485qOYA3 T4PwRqyJPV1yCPPpcBuZcSfeGFRadOdiHE1P3Ir1EIJQb6NWmgEE98miwhslidJpRk fAGSsk31XECtg== Date: Sun, 15 Jan 2023 16:29:04 +0000 From: Conor Dooley To: Jisheng Zhang Cc: Paul Walmsley , Palmer Dabbelt , Albert Ou , Anup Patel , Atish Patra , Heiko Stuebner , Conor Dooley , Andrew Jones , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvm-riscv@lists.infradead.org Subject: Re: [PATCH v4 06/13] riscv: introduce riscv_has_extension_[un]likely() Message-ID: References: <20230115154953.831-1-jszhang@kernel.org> <20230115154953.831-7-jszhang@kernel.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="HUUCBRPY9dF6efL2" Content-Disposition: inline In-Reply-To: <20230115154953.831-7-jszhang@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --HUUCBRPY9dF6efL2 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Hey Jisheng, On Sun, Jan 15, 2023 at 11:49:46PM +0800, Jisheng Zhang wrote: > Generally, riscv ISA extensions are fixed for any specific hardware > platform, so a hart's features won't change after booting. This > chacteristic makes it straightforward to use a static branch to check > if a specific ISA extension is supported or not to optimize > performance. >=20 > However, some ISA extensions such as SVPBMT and ZICBOM are handled > via. the alternative sequences. >=20 > Basically, for ease of maintenance, we prefer to use static branches > in C code, but recently, Samuel found that the static branch usage in > cpu_relax() breaks building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]. As > Samuel pointed out, "Having a static branch in cpu_relax() is > problematic because that function is widely inlined, including in some > quite complex functions like in the VDSO. A quick measurement shows > this static branch is responsible by itself for around 40% of the jump > table." >=20 > Samuel's findings pointed out one of a few downsides of static branches > usage in C code to handle ISA extensions detected at boot time: > static branch's metadata in the __jump_table section, which is not > discarded after ISA extensions are finalized, wastes some space. >=20 > I want to try to solve the issue for all possible dynamic handling of > ISA extensions at boot time. Inspired by Mark[2], this patch introduces > riscv_has_extension_*() helpers, which work like static branches but > are patched using alternatives, thus the metadata can be freed after > patching. >=20 > Link: https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@s= holland.org/ [1] > Link: https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-m= ark.rutland@arm.com/ [2] > Signed-off-by: Jisheng Zhang > Reviewed-by: Andrew Jones It'd be great, if, in the future, you would hold off on sending new versions of patchsets where the previous version is still being discussed [3]. ~3 days between versions is not very much, especially when that includes a weekend! I know you replied there earlier today with your opinion, but please give people a chance to read and respond, before resubmitting, so as not to split discussion between several threads. 3 - https://lore.kernel.org/linux-riscv/2398293.3Lj2Plt8kZ@diego/ Thanks, Conor. --HUUCBRPY9dF6efL2 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQRh246EGq/8RLhDjO14tDGHoIJi0gUCY8Qp0AAKCRB4tDGHoIJi 0pPnAQCsIsNUGq5IDZkIgbOznwtHyiQL3aKdAxLmntEOJDBOJgD/dvpGPPetV8gc DnsHNxe/93fMhhgnj9EtBjFUkmIeug0= =naIL -----END PGP SIGNATURE----- --HUUCBRPY9dF6efL2--