From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 283A23D3B8 for ; Sun, 27 Apr 2025 09:50:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745747408; cv=none; b=U0VNpJO960OkyOPebSUbhIIvLnDFQz2+y+a4LkwWTI29Nl/kVkwVOxfQy+CPLhGwjeQPufHHkKhtXKhfl7w1fXg/eu3QxYzF9neLWkpyhmihOm1FAyx8F+KuWPNMMmBpj4xxWsBvg2MNntqwgZ0CVOcnZscsUZ6te+AeVTRq0mM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745747408; c=relaxed/simple; bh=8UYVrDmaKV6ZZ3qAklvl94/hVHJqh5GIwBuL4dIJJTU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=px/87gRzwQQbk6+Ym7VV10YfwU1kP9C+6D0xI4O3nUP83qetIZw+yhTHJgtu5jIneC10+CBn24vhv3R7RLZtpyVJRnNwRmbt4FykSn7yk0+jodpQ9oPKYXsU+babKBSltykWlenpLhAvoqO7nrwn2LPfnh5IS2HqHwDvJGki5/M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LNUFh+HV; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LNUFh+HV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E2390C4CEE3; Sun, 27 Apr 2025 09:50:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1745747407; bh=8UYVrDmaKV6ZZ3qAklvl94/hVHJqh5GIwBuL4dIJJTU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=LNUFh+HV+LRgnlPShS8rXS7sdSbnkm7jbQkvCdQsHl+SntI9iCfZu/sITsPyhbxQP VxVG7pCTFrxhj2lsgxbjty+QQQUfryr9eRA9koIa29U19wCpHAhZZUoMF3g6dNaziW Nv9RG226Cx91ZCmPX1xL1EB7VB9OTb3sTaIPT+9QrLCaJAjylLeH4tqzse919IMCEn dLPiARw0uHyg45zc9QUJBD/RbN4rIYBlZ+o/PxX55rbbFkJciSe3Jau7DINL50lONy 6UzTYjOH4/st87qenWg4Mdd8Qo7nZjJlMjKVAK2vsoz6KgS/EbnH/Ea/txXkSFrGSP 8Wt7G1FYKHkCQ== Date: Sun, 27 Apr 2025 11:50:01 +0200 From: Ingo Molnar To: Linus Torvalds Cc: Arnd Bergmann , Arnd Bergmann , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Juergen Gross , Boris Ostrovsky , Alexander Usyskin , Greg Kroah-Hartman , Mateusz =?utf-8?Q?Jo=C5=84czyk?= , Mike Rapoport , Ard Biesheuvel , Peter Zijlstra , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org Subject: Re: [PATCH] [RFC] x86/cpu: rework instruction set selection Message-ID: References: <20250425141740.734030-1-arnd@kernel.org> 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=us-ascii Content-Disposition: inline In-Reply-To: * Linus Torvalds wrote: > On Sat, 26 Apr 2025 at 11:59, Arnd Bergmann wrote: > > > > Right. With the current set of features, CMOV is almost the > > same as 686. My reasoning was that support for CMOV has a > > very clear definition, with the instruction either being > > available or not. > > Yeah, I don't think there's any reason to make CMOV a reason to drop support. > > It has questionable performance impact - I doubt anybody can measure > it - and the "maintenance burden" is basically a single compiler > flag. > > (And yes, one use in a x86 header file that is pretty questionable > too: I think the reason for the cmov is actually i486-only behavior > and we could probably unify the 32-bit and 64-bit implementation) > > Let's not drop Pentium support due to something as insignificant as > that. Agreed on that. Idea to require CMOV dropped. Note that the outcome of 486 removal will likely be that the few remaining community distros that still offer x86-32 builds are either already 686-CMOV-only (Debian), or are going to drop their 486 builds and keep their 686-CMOV-only builds (Gentoo and Archlinux32) by way of simple inertia. (There's an off chance that they'll change their 486 builds to 586, but I think dropping the extra complication and standardizing on 686 will be the most likely outcome.) No commercial distro builds x86-32 with a modern v6.x series kernel AFAICS. Anyway, I agree that the maintenance cost on the kernel side to build non-CMOV kernels is very low. Thanks, Ingo