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=-6.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS 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 AF682C11D05 for ; Thu, 20 Feb 2020 15:06:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8371D208E4 for ; Thu, 20 Feb 2020 15:06:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582211164; bh=qZGMqmf13kGIaYi/07SmSX1RGyMwOKwiwho8FB8lkTs=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=vsC4lEAoO8XocwOIiMcBVhd7oHXMJWJe0RQPwynIJAbVuMI28K45avOkwXgyl5PHB b/WiYzPkepWhf1e3T7Q8cmh+raf5NrJXtLNSactn1iOA6ZPx57aBH2pw7b28bUSZUn 7aEULy9dW+jNyxU3HyPwrpUFKTTH0qhRYvEoklDc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728388AbgBTPGD (ORCPT ); Thu, 20 Feb 2020 10:06:03 -0500 Received: from mail.kernel.org ([198.145.29.99]:59538 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728133AbgBTPGC (ORCPT ); Thu, 20 Feb 2020 10:06:02 -0500 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id F14FA2071E; Thu, 20 Feb 2020 15:06:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582211162; bh=qZGMqmf13kGIaYi/07SmSX1RGyMwOKwiwho8FB8lkTs=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=NryJuAResREVCHM8mQNQCcamlRvxhjbRJxG0YuRw7rtC44JpxQ5ZDgw9dU/I10TDI DvepeERBJ3zq76TvxDDwTS+DVnXwkyVH0SAF6bhnilPh4OQrZKvVTIYw4ROgIOmOHk rNIQ1sC5bmZKTPVzN1InK+HWplCsUgImxfL5KERE= Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1j4nOq-006km6-8n; Thu, 20 Feb 2020 15:06:00 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Thu, 20 Feb 2020 15:06:00 +0000 From: Marc Zyngier To: minyard@acm.org Cc: Will Deacon , Catalin Marinas , linux-arm-kernel@lists.infradead.org, Corey Minyard , linux-kernel@vger.kernel.org, Corey Minyard Subject: Re: [PATCH v2] arm64:kgdb: Fix kernel single-stepping In-Reply-To: <20200220145048.GH3704@minyard.net> References: <20200219152403.3495-1-minyard@acm.org> <1416dca51b52dff349923184f41d48e8@kernel.org> <20200220145048.GH3704@minyard.net> Message-ID: <375815af3c711b94dd2ee56326c2dd3b@kernel.org> X-Sender: maz@kernel.org User-Agent: Roundcube Webmail/1.3.10 X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: minyard@acm.org, will@kernel.org, catalin.marinas@arm.com, linux-arm-kernel@lists.infradead.org, cminyard@mvista.com, linux-kernel@vger.kernel.org, tcminyard@gmail.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-02-20 14:50, Corey Minyard wrote: > On Thu, Feb 20, 2020 at 02:21:36PM +0000, Marc Zyngier wrote: >> On 2020-02-19 15:24, minyard@acm.org wrote: >> > From: Corey Minyard >> >> [...] >> >> > After studying the EL0 handling for this, I realized an issue with using >> > MDSCR to check if single step is enabled: it can be expensive on a VM. >> > So check the task flag first to see if single step is enabled. Then >> > check MDSCR if the task flag is set. >> >> Very tangential remark: I'd really like people *not* to try and >> optimize >> Linux based on the behaviour of a hypervisor. In general, reading a >> system register is fast, and the fact that it traps on a given >> hypervisor >> at some point may not be true in the future, nor be a valid assumption >> across hypervisors. > > Normally I would agree, but I based this upon git commit > https://github.com/torvalds/linux/commit/2a2830703a2371b47f7b50b1d35cb15dc0e2b717 > which seemed to say that it was a significant enough factor to do in > the > EL0 case. And that's a blast from a distant past. Hypervisors have changed drastically over these 6 years, and I'm still sitting on a bunch of patches that *could* change the way MDSCR_EL1 is handled. M. -- Jazz is not dead. It just smells funny...