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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 476A6E732EA for ; Thu, 28 Sep 2023 16:55:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231571AbjI1Qz4 (ORCPT ); Thu, 28 Sep 2023 12:55:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59294 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231263AbjI1Qzy (ORCPT ); Thu, 28 Sep 2023 12:55:54 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 7F6F71A3 for ; Thu, 28 Sep 2023 09:55:52 -0700 (PDT) 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 829881FB; Thu, 28 Sep 2023 09:56:30 -0700 (PDT) Received: from [10.1.32.180] (e126864.cambridge.arm.com [10.1.32.180]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 90DAA3F5A1; Thu, 28 Sep 2023 09:55:49 -0700 (PDT) Message-ID: <6687f58c-0da9-0583-2dc1-2089f292b745@arm.com> Date: Thu, 28 Sep 2023 17:55:39 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v2 0/2] KVM: arm64: Support for Arm v8.8 memcpy instructions in KVM guests Content-Language: en-US To: Oliver Upton Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, Marc Zyngier , James Morse , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Vladimir Murzin , Colton Lewis , linux-kernel@vger.kernel.org References: <20230922112508.1774352-1-kristina.martsenko@arm.com> From: Kristina Martsenko In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27/09/2023 07:00, Oliver Upton wrote: > Hi Kristina, Hi Oliver, > > On Fri, Sep 22, 2023 at 12:25:06PM +0100, Kristina Martsenko wrote: >> Hi, >> >> This is v2 of the series to allow using the new Arm memory copy instructions >> in KVM guests. See v1 for more information [1]. > > > Thanks for sending out the series. I've been thinking about what the > architecture says for MOPS, and I wonder if what's currently in the > Arm ARM is clear enough for EL1 software to be written robustly. > > While HCRX_EL2.MCE2 allows the hypervisor to intervene on MOPS > exceptions from EL1, there's no such control for EL0. So when vCPU > migration occurs EL1 could get an unexpected MOPS exception, even for a > process that was pinned to a single (virtual) CPU implementation. > > Additionally, the wording of I_NXHPS seems to suggest that EL2 handling > of MOPS exceptions is only expected in certain circumstances where EL1 is > incapable of handling an exception. Is the unwritten expectation then > that EL1 software should tolerate 'unexpected' MOPS exceptions from EL1 > and EL0, even if EL1 did not migrate the PE context? > > Perhaps I'm being pedantic, but I'd really like for there to be some > documentation that suggests MOPS exceptions can happen due to context > migration done by a higher EL as that is the only option in the context > of virtualization. That's a good point. This shouldn't affect Linux guests as Linux is always able to handle a MOPS exception coming from EL0. But it would affect any non-Linux guest that pins all its EL0 tasks and doesn't implement a handler. It's not clear to me what the expectation for guests is, I'll ask the architects to clarify and get back to you. Thanks for the feedback! Kristina