From mboxrd@z Thu Jan 1 00:00:00 1970 From: catalin.marinas@arm.com (Catalin Marinas) Date: Wed, 8 Aug 2018 13:34:09 +0100 Subject: [PATCH] arm64: Trap WFI executed in userspace In-Reply-To: <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com> References: <20180807093326.5090-1-marc.zyngier@arm.com> <20180807100437.GA9097@e103592.cambridge.arm.com> <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com> Message-ID: <20180808123408.GC24736@iMac.local> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, Aug 07, 2018 at 11:24:34AM +0100, Marc Zyngier wrote: > On 07/08/18 11:05, Dave Martin wrote: > > On Tue, Aug 07, 2018 at 10:33:26AM +0100, Marc Zyngier wrote: > >> It recently came to light that userspace can execute WFI, and that > >> the arm64 kernel doesn trap this event. This sounds rather benign, Nitpick: "doesn't". > >> but the kernel should decide when it wants to wait for an interrupt, > >> and not userspace. > >> > >> Let's trap WFI and treat it as a way to yield the CPU to another > >> process. [...] > > I can't think of a legitimate reason for userspace to execute WFI > > however. Userspace doesn't have interrupts under Linux, so it makes > > no sense to wait for one. > > > > Have we seen anybody using WFI in userspace? It may be cleaner to > > map this to SIGILL rather than be permissive and regret it later. > > I couldn't find any user, and I'm happy to just send userspace to hell > in that case. But it could also been said that since it was never > prevented, it is a de-facto ABI. I wouldn't really go as far as SIGILL on WFI. I think the patch is fine as it is. In case Will plans to merge it: Acked-by: Catalin Marinas 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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 DAF71C4646D for ; Wed, 8 Aug 2018 12:34:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9F753219DB for ; Wed, 8 Aug 2018 12:34:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9F753219DB Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727229AbeHHOxn (ORCPT ); Wed, 8 Aug 2018 10:53:43 -0400 Received: from foss.arm.com ([217.140.101.70]:38116 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726733AbeHHOxm (ORCPT ); Wed, 8 Aug 2018 10:53:42 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2552A7A9; Wed, 8 Aug 2018 05:34:13 -0700 (PDT) Received: from iMac.local (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 112663F5D0; Wed, 8 Aug 2018 05:34:11 -0700 (PDT) Date: Wed, 8 Aug 2018 13:34:09 +0100 From: Catalin Marinas To: Marc Zyngier Cc: Dave Martin , Will Deacon , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] arm64: Trap WFI executed in userspace Message-ID: <20180808123408.GC24736@iMac.local> References: <20180807093326.5090-1-marc.zyngier@arm.com> <20180807100437.GA9097@e103592.cambridge.arm.com> <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9af8bb9a-7c6c-2560-5965-118dfadf8141@arm.com> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 07, 2018 at 11:24:34AM +0100, Marc Zyngier wrote: > On 07/08/18 11:05, Dave Martin wrote: > > On Tue, Aug 07, 2018 at 10:33:26AM +0100, Marc Zyngier wrote: > >> It recently came to light that userspace can execute WFI, and that > >> the arm64 kernel doesn trap this event. This sounds rather benign, Nitpick: "doesn't". > >> but the kernel should decide when it wants to wait for an interrupt, > >> and not userspace. > >> > >> Let's trap WFI and treat it as a way to yield the CPU to another > >> process. [...] > > I can't think of a legitimate reason for userspace to execute WFI > > however. Userspace doesn't have interrupts under Linux, so it makes > > no sense to wait for one. > > > > Have we seen anybody using WFI in userspace? It may be cleaner to > > map this to SIGILL rather than be permissive and regret it later. > > I couldn't find any user, and I'm happy to just send userspace to hell > in that case. But it could also been said that since it was never > prevented, it is a de-facto ABI. I wouldn't really go as far as SIGILL on WFI. I think the patch is fine as it is. In case Will plans to merge it: Acked-by: Catalin Marinas