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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 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 31E99C433DB for ; Thu, 4 Mar 2021 00:30:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 03BD364EEC for ; Thu, 4 Mar 2021 00:30:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1451099AbhCDA3z (ORCPT ); Wed, 3 Mar 2021 19:29:55 -0500 Received: from foss.arm.com ([217.140.110.172]:53272 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233875AbhCCRgy (ORCPT ); Wed, 3 Mar 2021 12:36:54 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 57FC131B; Wed, 3 Mar 2021 09:36:02 -0800 (PST) Received: from slackpad.fritz.box (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 894263F7D7; Wed, 3 Mar 2021 09:36:01 -0800 (PST) Date: Wed, 3 Mar 2021 17:35:57 +0000 From: Andre Przywara To: Alexandru Elisei Cc: drjones@redhat.com, kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu Subject: Re: [kvm-unit-tests PATCH 1/6] arm64: Remove unnecessary ISB when writing to SPSel Message-ID: <20210303173557.0626667d@slackpad.fritz.box> In-Reply-To: <20210227104201.14403-2-alexandru.elisei@arm.com> References: <20210227104201.14403-1-alexandru.elisei@arm.com> <20210227104201.14403-2-alexandru.elisei@arm.com> Organization: Arm Ltd. X-Mailer: Claws Mail 3.17.1 (GTK+ 2.24.31; x86_64-slackware-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Sat, 27 Feb 2021 10:41:56 +0000 Alexandru Elisei wrote: > Software can use the SPSel operand to write directly to PSTATE.SP. > According to ARM DDI 0487F.b, page D1-2332, writes to PSTATE are > self-synchronizing and no ISB is needed: > > "Writes to the PSTATE fields have side-effects on various aspects of the PE > operation. All of these side-effects are guaranteed: > - Not to be visible to earlier instructions in the execution stream. > - To be visible to later instructions in the execution stream." > > Signed-off-by: Alexandru Elisei I am always a bit wary about *removing* barriers, but I can confirm that the ARM ARM indeed makes this guarantee above, and SP access sounds like an easy enough case, so: Reviewed-by: Andre Przywara Cheers, Andre > --- > arm/cstart64.S | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/arm/cstart64.S b/arm/cstart64.S > index 0428014aa58a..fc1930bcdb53 100644 > --- a/arm/cstart64.S > +++ b/arm/cstart64.S > @@ -54,7 +54,6 @@ start: > /* set up stack */ > mov x4, #1 > msr spsel, x4 > - isb > adrp x4, stackptr > add sp, x4, :lo12:stackptr >