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,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 CD9A3C46470 for ; Tue, 7 Aug 2018 10:31:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 74F23208B6 for ; Tue, 7 Aug 2018 10:31:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 74F23208B6 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 S1727408AbeHGMoi (ORCPT ); Tue, 7 Aug 2018 08:44:38 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:52128 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727149AbeHGMoi (ORCPT ); Tue, 7 Aug 2018 08:44:38 -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 0939980D; Tue, 7 Aug 2018 03:30:58 -0700 (PDT) Received: from e103592.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EC1D83F2EA; Tue, 7 Aug 2018 03:30:56 -0700 (PDT) Date: Tue, 7 Aug 2018 11:30:54 +0100 From: Dave Martin To: Marc Zyngier Cc: Catalin Marinas , Will Deacon , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] arm64: Trap WFI executed in userspace Message-ID: <20180807103054.GB9097@e103592.cambridge.arm.com> 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/1.5.23 (2014-03-12) 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, > >> 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. > > > > This doesn't amount to a justification. > > > > If the power controller is unexpectedly left in a bad state so that > > WFI will do something nasty to a cpu that may enter userspace, then we > > probably have bigger problems. > > > > So, maybe it really is pretty harmless to let userspace execute this. > > Or not. It is also a very good way for userspace to find out when an > interrupt gets delivered and start doing all kind of probing on the > kernel. The least the userspace knows about that, the better I feel. Possibly. I suspect there are other ways to guess pretty accurately when an interrupt occurs, but WFI allows greater precision. > > 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. Agreed. I wonder whether it's sufficient to have this mapping to SIGILL in -next for a while and see whether anybody complains. Cheers ---Dave