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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no 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 E0D0FC3A5A5 for ; Thu, 5 Sep 2019 09:15:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B524521848 for ; Thu, 5 Sep 2019 09:15:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567674955; bh=WmqYVKOOYA94qGvoGgu669eTuZYpqzg+UGzpOtosKGQ=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=uj5FeBw9c+dEtymKkygajMLH0Ib6U2cODBHpO05Nra2rBlt8dUgenmbhbqlVN78vh spylqTcVpgDXrACQYW6QonxdN8yXmcqHji+Fk6PkX0/z0PriITROCddj9Bqdc2jMJT Gkl1jShWxFatTrsRZ0gUvtAitjPBf/8NPa5Yki+c= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732035AbfIEJPy (ORCPT ); Thu, 5 Sep 2019 05:15:54 -0400 Received: from foss.arm.com ([217.140.110.172]:39936 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731109AbfIEJPy (ORCPT ); Thu, 5 Sep 2019 05:15:54 -0400 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 89DE9360; Thu, 5 Sep 2019 02:15:53 -0700 (PDT) Received: from big-swifty.misterjones.org (unknown [10.1.27.38]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1612E3F67D; Thu, 5 Sep 2019 02:15:50 -0700 (PDT) Date: Thu, 05 Sep 2019 10:15:49 +0100 Message-ID: <86k1anrtmi.wl-maz@kernel.org> From: Marc Zyngier To: Peter Maydell Cc: Heinrich Schuchardt , James Morse , Julien Thierry , Suzuki K Pouloze , Stefan Hajnoczi , =?UTF-8?B?IkRhbmllbCBQIC4gQmVycmFuZ8OpIg==?= , arm-mail-list , kvmarm@lists.cs.columbia.edu, lkml - Kernel Mailing List Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded In-Reply-To: References: <20190904180736.29009-1-xypron.glpk@gmx.de> <86r24vrwyh.wl-maz@kernel.org> <86mufjrup7.wl-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL/10.8 EasyPG/1.0.0 Emacs/26 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) Organization: Approximate MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 05 Sep 2019 09:56:44 +0100, Peter Maydell wrote: > > On Thu, 5 Sep 2019 at 09:52, Marc Zyngier wrote: > > > > On Thu, 05 Sep 2019 09:16:54 +0100, > > Peter Maydell wrote: > > > This is true, but the problem is that barfing out to userspace > > > makes it harder to debug the guest because it means that > > > the VM is immediately destroyed, whereas AIUI if we > > > inject some kind of exception then (assuming you're set up > > > to do kernel-debug via gdbstub) you can actually examine > > > the offending guest code with a debugger because at least > > > your VM is still around to inspect... > > > > To Christoffer's point, I find the benefit a bit dubious. Yes, you get > > an exception, but the instruction that caused it may be completely > > legal (store with post-increment, for example), leading to an even > > more puzzled developer (that exception should never have been > > delivered the first place). > > Right, but the combination of "host kernel prints a message > about an unsupported load/store insn" and "within-guest debug > dump/stack trace/etc" is much more useful than just having > "host kernel prints message" and "QEMU exits"; and it requires > about 3 lines of code change... Which is wrong, and creates a new behaviour that isn't specified anywhere. > > > I'm far more in favour of dumping the state of the access in the run > > structure (much like we do for a MMIO access) and let userspace do > > something about it (such as dumping information on the console or > > breaking). It could even inject an exception *if* the user has asked > > for it. > > ...whereas this requires agreement on a kernel-userspace API, > larger changes in the kernel, somebody to implement the userspace > side of things, and the user to update both the kernel and QEMU. > It's hard for me to see that the benefit here over the 3-line > approach really outweighs the extra effort needed. 3 lines that already require the host kernel to be updated, and create a legacy that we'll never be able to get rid of. > In practice saying "we should do this" is saying "we're going to do > nothing", based on the historical record. Thanks for the vote of confidence... M. -- Jazz is not dead, it just smells funny.