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 ADCBFC46467 for ; Wed, 18 Jan 2023 22:18:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229616AbjARWSb (ORCPT ); Wed, 18 Jan 2023 17:18:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60670 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229446AbjARWSM (ORCPT ); Wed, 18 Jan 2023 17:18:12 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3EEA24A21D; Wed, 18 Jan 2023 14:18:11 -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 dfw.source.kernel.org (Postfix) with ESMTPS id C1ED561A5F; Wed, 18 Jan 2023 22:18:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8D857C433D2; Wed, 18 Jan 2023 22:18:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674080290; bh=OXPGiUZ4pwYxZy+VWAnIylIre/fE0IeCBH5Rt/zzbso=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Ayu1+rIsdsFAO+vgB7460xeODdc0u9LDm3VB+lTV9RVxpihcqQE6MGoywCdRmVp/v cvQj1a0wQHAtJGHwmAyLkHZB0xxRqTQ4TmLi8LeBShjs3A6jxaxzbYjq7WPg2NTO0o 8jkVOYSY2A3P+J0XHtWDDNYln+o/d+HugHhekRngFybMrrmc1AVTmLfSlwxwtXKVGh Bqiga5YfDRKneLVt1p3VjReM2IZrktJ881Fyy+lme/lbWyDWqZBoMi7maCDt+nXs/v sTuOPXDXCwuD+v6UpAuNjbrbDKTUZRJ2MfCIbm8lMJsHgjM3LZPrSikKH8vD48MCRy FM+z70t4d+ZOw== Date: Wed, 18 Jan 2023 22:18:05 +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="Tsh+vw9Kkt0iE9EC" Content-Disposition: inline In-Reply-To: <20230115154953.831-7-jszhang@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --Tsh+vw9Kkt0iE9EC Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 I think I just didn't leave an R-b last time because of the ongoing discussion & the functions naming. Given the conversation I attempted to summarise at [1], I'm satisfied that we're not just introducing something that'd conflict with trying to use this mechanism for non-extension capabilities: Acked-by: Conor Dooley 1 - https://lore.kernel.org/linux-riscv/Y8hqptFcUgjhns4F@spud/ > --- > arch/riscv/include/asm/hwcap.h | 37 ++++++++++++++++++++++++++++++++++ > 1 file changed, 37 insertions(+) >=20 > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwca= p.h > index 09a7767723f6..1767a9ce1a04 100644 > --- a/arch/riscv/include/asm/hwcap.h > +++ b/arch/riscv/include/asm/hwcap.h > @@ -8,6 +8,7 @@ > #ifndef _ASM_RISCV_HWCAP_H > #define _ASM_RISCV_HWCAP_H > =20 > +#include > #include > #include > #include > @@ -97,6 +98,42 @@ static __always_inline int riscv_isa_ext2key(int num) > } > } > =20 > +static __always_inline bool > +riscv_has_extension_likely(const unsigned long ext) > +{ > + compiletime_assert(ext < RISCV_ISA_EXT_MAX, > + "ext must be < RISCV_ISA_EXT_MAX"); > + > + asm_volatile_goto( > + ALTERNATIVE("j %l[l_no]", "nop", 0, %[ext], 1) > + : > + : [ext] "i" (ext) > + : > + : l_no); > + > + return true; > +l_no: > + return false; > +} > + > +static __always_inline bool > +riscv_has_extension_unlikely(const unsigned long ext) > +{ > + compiletime_assert(ext < RISCV_ISA_EXT_MAX, > + "ext must be < RISCV_ISA_EXT_MAX"); > + > + asm_volatile_goto( > + ALTERNATIVE("nop", "j %l[l_yes]", 0, %[ext], 1) > + : > + : [ext] "i" (ext) > + : > + : l_yes); > + > + return false; > +l_yes: > + return true; > +} > + > unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap); > =20 > #define riscv_isa_extension_mask(ext) BIT_MASK(RISCV_ISA_EXT_##ext) > --=20 > 2.38.1 >=20 >=20 > _______________________________________________ > linux-riscv mailing list > linux-riscv@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-riscv --Tsh+vw9Kkt0iE9EC Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQRh246EGq/8RLhDjO14tDGHoIJi0gUCY8hwHAAKCRB4tDGHoIJi 0mPWAQDp9Xg+dw4SJ1uKG3EHoxsZNjoaSkE7qWtOkI+lJXFysgEAmiWO4UEYdPA4 2X07QMBRd1wVUcth/t2sQgBu+yhYugE= =zMjo -----END PGP SIGNATURE----- --Tsh+vw9Kkt0iE9EC--