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 X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D1D5BC433DF for ; Tue, 13 Oct 2020 16:59:20 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id EDADB25307 for ; Tue, 13 Oct 2020 16:59:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linaro.org header.i=@linaro.org header.b="rN1o9EBg" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EDADB25307 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linaro.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:41126 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kSNdt-00006f-Ni for qemu-devel@archiver.kernel.org; Tue, 13 Oct 2020 12:59:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:58846) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kSNdD-0007zL-Rn for qemu-devel@nongnu.org; Tue, 13 Oct 2020 12:58:35 -0400 Received: from mail-pj1-x1044.google.com ([2607:f8b0:4864:20::1044]:35886) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1kSNdC-0006fl-C7 for qemu-devel@nongnu.org; Tue, 13 Oct 2020 12:58:35 -0400 Received: by mail-pj1-x1044.google.com with SMTP id a1so242286pjd.1 for ; Tue, 13 Oct 2020 09:58:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:content-language:content-transfer-encoding; bh=kjcdsLCEJCFzeMrTcX3WjkgAnDjuz9bXPb1/3p7Dwoc=; b=rN1o9EBgVAlbdP950rNMWE+oFMxSUkYsQbgTEGl2hdhRTIAzfNXnemaCqTPxoq8XUK ZuQx31NfM4A5U4nZIBlh0VAdUt7I676zLu3N8yqnP/P/D5HMFTRXGudKNthQ+UonPmFf yH5yx40lO809dOKU00kJlDE5y02kClvoERCbXfOL+Nzb8zEYT4t7KwnaK2Cq6Jr5TitI 1fLi8RXn5kPcLDg8ysGdEEoJ7x2GYuEHCSV+70/Tv640ynoZ6xpLtudbGadNFfthXeJz WDD3hcK72jDX4POsA/tZ7MXjZLAWBwRmEGdlVpG8YBI+uCNMpd02ajcw1bSG2YQj/XqN sm8Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=kjcdsLCEJCFzeMrTcX3WjkgAnDjuz9bXPb1/3p7Dwoc=; b=tjCzrwNgZK7FeIyzsYmnyNLPMgUDothx2ePGLMThnBHcGVDO5SNy3hzItuiLxypdyP Ij2o8TDXudmgN2y45QwYbIaa7VcK1z4R1LgHFwcH8wr+25b+1plvwxVI1aTdSgNjJCkw UH2loNSQhRozFAbxUBQK+fkb0qphDhQHcV+WACItTgvEmpSlc2kcDRrWsbtM/uA6wzhd OR+luwJfxfyYgjd47eK6liPlGiTylGgJjeCD0UQic0Xv11xSWji6kAg/nhqJ5W+R3A71 ZWg8HFc0ciR4QktoShBbOTzYujD+879MyemAQDarNDl40R9UBJKzU9usUUFTeMr6U6yM XyDA== X-Gm-Message-State: AOAM532FoksSlPHvxesLHIGCf0fWFypaGJJcgJSq+vTl9LJhm3ddvMeT WvtbTG3N43c1FuGAiZKkuHFBlCEGz77IcP9P X-Google-Smtp-Source: ABdhPJzhpIdyaNUrN2O/SIeqjgz12MWbdzC9pdBWtPZ8UF7pnYGwA7ZKyawcXHnOk9z3k0M5lP+hAQ== X-Received: by 2002:a17:90a:bf05:: with SMTP id c5mr657235pjs.11.1602608312296; Tue, 13 Oct 2020 09:58:32 -0700 (PDT) Received: from [192.168.1.11] ([71.212.141.89]) by smtp.gmail.com with ESMTPSA id x25sm178339pfr.132.2020.10.13.09.58.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 Oct 2020 09:58:31 -0700 (PDT) Subject: Re: [PATCH 06/10] target/arm: Implement v8.1M branch-future insns (as NOPs) To: Peter Maydell , qemu-arm@nongnu.org, qemu-devel@nongnu.org References: <20201012153746.9996-1-peter.maydell@linaro.org> <20201012153746.9996-7-peter.maydell@linaro.org> From: Richard Henderson Message-ID: Date: Tue, 13 Oct 2020 09:58:29 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20201012153746.9996-7-peter.maydell@linaro.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::1044; envelope-from=richard.henderson@linaro.org; helo=mail-pj1-x1044.google.com X-detected-operating-system: by eggs.gnu.org: No matching host in p0f cache. That's all we know. X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 10/12/20 8:37 AM, Peter Maydell wrote: > v8.1M implements a new 'branch future' feature, which is a > set of instructions that request the CPU to perform a branch > "in the future", when it reaches a particular execution address. > In hardware, the expected implementation is that the information > about the branch location and destination is cached and then > acted upon when execution reaches the specified address. > However the architecture permits an implementation to discard > this cached information at any point, and so guest code must > always include a normal branch insn at the branch point as > a fallback. In particular, an implementation is specifically > permitted to treat all BF insns as NOPs (which is equivalent > to discarding the cached information immediately). > > For QEMU, implementing this caching of branch information > would be complicated and would not improve the speed of > execution at all, so we make the IMPDEF choice to implement > all BF insns as NOPs. > > Signed-off-by: Peter Maydell Reviewed-by: Richard Henderson r~