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[23.128.96.18]) by mx.google.com with ESMTP id co11si11497636edb.280.2021.03.15.16.27.01; Mon, 15 Mar 2021 16:27:24 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b="eGXMk4/O"; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231821AbhCOSYZ (ORCPT + 99 others); Mon, 15 Mar 2021 14:24:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46140 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229979AbhCOSYG (ORCPT ); Mon, 15 Mar 2021 14:24:06 -0400 Received: from mail-io1-xd29.google.com (mail-io1-xd29.google.com [IPv6:2607:f8b0:4864:20::d29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A3F40C06174A; Mon, 15 Mar 2021 11:24:06 -0700 (PDT) Received: by mail-io1-xd29.google.com with SMTP id m7so15389420iow.7; Mon, 15 Mar 2021 11:24:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:reply-to:from:date:message-id :subject:to:cc; bh=2NfTgo/n4iFp2Ay18SODmBMI1NAFsimcTVTCJTYL8F4=; b=eGXMk4/Oo5giwVYy/oKwWnyTCAtBcuQLBZG7qDyNF0yEFz5p897qUmS/2xbu9UZw8j CwYhPAyw010jSshN2tIO6UHGEiv5vbhhieL17XtGE8jBvpymh2SCuqyH5b/N1bWuXP6O rYIlvucyMhR0wOQGQeb7XYgK6+m/5av3+7EXgwagkAVyvV+osBIpPtiSn4PFT5RYCsg/ lEetIflo3jt2Y+zZ3C7w1+SY/A++YKyr+u1MRSsLp741lQoHkvE30gePiainiqAg1cTw WNlizd5MzH/42kX+jFPR3u4/84O2FsWNKQCPYVC3lF3Z90fmX1DbDvpmQUX0VyVJHfhh FnPg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:reply-to :from:date:message-id:subject:to:cc; bh=2NfTgo/n4iFp2Ay18SODmBMI1NAFsimcTVTCJTYL8F4=; b=qUBRvw+yuacxc8joQ6oIbnRXF71kxut/zMD8FFqk0kBVkNMukJIremv5a6+xo0J4Qz jMReRwgqjNx4ztfiRVoIn+aAdLK7i+5ZbsC8N9KKp+c+ujclNmQGoNhofBuqwUHVB0Eu Nqlx0G3tXcYLW1D8rMKINGLUnI71u79+6LnTycGwhCvscHbHRNqzgrS6Pr7cgU8CTB9U chp3fyrHyER8Rib2W9VmH6grkt8AFxbsoGZNJGlW37VCQbnmXrHWU60VJn/S3znmGaOE MJ+WQcP6Vss1jmhOas4Yw4/iww7jG1+fyKY+oij5vzVsCuPHWy0DBW3e2+NQqwuzVFlL YA0Q== X-Gm-Message-State: AOAM532kZAl5E4Ltrb0L33eO3AaBCJLuGr9uglHRWd7+DjfX+tWTdnR9 ZVmdYdWzPaXxdGq7WXbCpgpSBCWa8GS3aO5SllHFW4d/u7LiEw== X-Received: by 2002:a02:74a:: with SMTP id f71mr10560582jaf.30.1615832646083; Mon, 15 Mar 2021 11:24:06 -0700 (PDT) MIME-Version: 1.0 References: <20210313084923.GA16144@zn.tnic> <20210313121541.GC16144@zn.tnic> <20210313124919.GD16144@zn.tnic> <20210313132927.GF16144@zn.tnic> In-Reply-To: Reply-To: sedat.dilek@gmail.com From: Sedat Dilek Date: Mon, 15 Mar 2021 19:23:29 +0100 Message-ID: Subject: Re: [PATCH 0/2] x86: Remove ideal_nops[] To: Peter Zijlstra Cc: Borislav Petkov , x86@kernel.org, rostedt@goodmis.org, hpa@zytor.com, torvalds@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-toolchains@vger.kernel.org, jpoimboe@redhat.com, alexei.starovoitov@gmail.com, mhiramat@kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 15, 2021 at 7:10 PM Peter Zijlstra wrote: > > On Mon, Mar 15, 2021 at 06:04:41PM +0100, Sedat Dilek wrote: > > > make V=1 -j4 LLVM=1 LLVM_IAS=1 > > So for giggles I checked, neither GCC nor LLVM seem to emit prefix NOPs > when building with -march=sandybridge, they always use MOPL. > > Furthermore, the kernel explicitly sets: -falign-jumps=1 > -falign-loops=1, which, when not specified, default to 16 or so. > > This means that your userspace is *littered* with NOPL, even when you > build your entire distro from source with -march=sandybridge. > (arch/gentoo FTW I suppose). > That reminds me of the Git repo of the wireguard maintainer. "x86: enable additional cpu optimizations for gcc v9.1+" You mean something like that ^^? - Sedat - [1] https://git.zx2c4.com/laptop-kernel/commit/?id=116badbe0a18bc36ba90acb8b80cff41f9ab0686 > (The only good new is that recent LLVM has a pass to use alternative > instruction encoding in order to grow a basic block in size in order to > minimize the amount of NOP it needs to emit at the end in order to > satisfy the jump/loop alignment.) > > So if you *really* deeply care about NOP performance on your SNB, start > by teaching LLVM about prefix NOPs and rebuild your complete userspace. > At that point, you can do some trivial patches to the kernel to make it > use -march=sandybridge and prefix NOPs too. > > Until that time, the vast majority of NOPs your CPU will execute will be > NOPL.