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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 79B93C433EF for ; Tue, 24 May 2022 09:48:11 +0000 (UTC) Received: from localhost ([::1]:50136 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ntR98-0004kF-Dz for qemu-devel@archiver.kernel.org; Tue, 24 May 2022 05:48:10 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41348) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ntQpB-0006rt-IO for qemu-devel@nongnu.org; Tue, 24 May 2022 05:27:33 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:53250) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ntQpA-0004jl-13 for qemu-devel@nongnu.org; Tue, 24 May 2022 05:27:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1653384451; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=H2Qqaef//XkcG+pw5lVlymXwThryn9QBEMiUR3fU8i8=; b=H8atpI406kxQXIhcc31JOkbpzuh4viVtfrO16TOHv26TnXeSPseTEUuBcIugYETWgXFbaQ TSGPybeJyS4wvMPvKLWt1VNjgjIU3l/VmQ0m10f1vGY0TkXIZHmD8rVodbYGmjcD+W7JVO NA8j+KxwpPfT7fYbJhrlBj+Plc5lmpQ= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-392-gU6ZymubOGS8llmuI856fw-1; Tue, 24 May 2022 05:27:27 -0400 X-MC-Unique: gU6ZymubOGS8llmuI856fw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5C19E3C025BB; Tue, 24 May 2022 09:27:27 +0000 (UTC) Received: from oldenburg.str.redhat.com (unknown [10.39.192.56]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 853ECC27E8F; Tue, 24 May 2022 09:27:26 +0000 (UTC) From: Florian Weimer To: Richard Henderson Cc: qemu-devel@nongnu.org Subject: Re: Emulating CPUs with larger atomic accesses References: <87k0apeor5.fsf@oldenburg.str.redhat.com> <37f4c866-4344-37ba-b64b-fd338dc96887@linaro.org> Date: Tue, 24 May 2022 11:27:24 +0200 In-Reply-To: <37f4c866-4344-37ba-b64b-fd338dc96887@linaro.org> (Richard Henderson's message of "Sat, 21 May 2022 18:07:33 -0700") Message-ID: <87sfozuvpf.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.85 on 10.11.54.8 Received-SPF: pass client-ip=170.10.133.124; envelope-from=fweimer@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 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" * Richard Henderson: > On 5/13/22 03:00, Florian Weimer wrote: >> What's QEMU's approach to emulating CPU instructions that atomatically >> operate on values larger than what is supported by the host CPU? >> I assume that for full system emulation, this is not a problem, but >> qemu-user will not achieve atomic behavior on shared memory mappings. >> How much of a problem is this in practice? > > Well, it doesn't work, no. In practice, x86_64 supports 128-bit > atomic operations, and guest requires more than that. No one really > cares anymore about 32-bit hosts with smaller atomic operations. Which part doesn't work? Full-system emulation? Do guests really require wider-than-128 atomics? That's quite surprising? Thanks, Florian