From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 49919 invoked by alias); 16 Jan 2017 13:32:39 -0000 Mailing-List: contact gdb-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sourceware.org Received: (qmail 49888 invoked by uid 89); 16 Jan 2017 13:32:38 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-5.1 required=5.0 tests=BAYES_00,RP_MATCHES_RCVD,SPF_PASS autolearn=ham version=3.3.2 spammy=tasks, ptrace, task's, read-only X-Spam-User: qpsmtpd, 2 recipients X-HELO: foss.arm.com Received: from foss.arm.com (HELO foss.arm.com) (217.140.101.70) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 16 Jan 2017 13:32:37 +0000 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 1383C707; Mon, 16 Jan 2017 05:32:36 -0800 (PST) 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 1C3C23F220; Mon, 16 Jan 2017 05:32:33 -0800 (PST) Date: Mon, 16 Jan 2017 13:32:00 -0000 From: Dave Martin To: Yao Qi Cc: linux-arch@vger.kernel.org, Florian Weimer , libc-alpha@sourceware.org, Ard Biesheuvel , Marc Zyngier , gdb@sourceware.org, Joseph Myers , Szabolcs Nagy , linux-arm-kernel@lists.infradead.org, Alan Hayward , Torvald Riegel , Christoffer Dall Subject: Re: [RFC PATCH 08/10] arm64/sve: ptrace: Wire up vector length control and reporting Message-ID: <20170116133231.GO3699@e103592.cambridge.arm.com> References: <1484220369-23970-1-git-send-email-Dave.Martin@arm.com> <1484220369-23970-9-git-send-email-Dave.Martin@arm.com> <20170116122038.GG28060@E107787-LIN> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170116122038.GG28060@E107787-LIN> User-Agent: Mutt/1.5.23 (2014-03-12) X-SW-Source: 2017-01/txt/msg00021.txt.bz2 On Mon, Jan 16, 2017 at 12:20:38PM +0000, Yao Qi wrote: > On 17-01-12 11:26:07, Dave Martin wrote: > > This patch adds support for manipulating a task's vector length at > > runtime via ptrace. > > > > I hope kernel doesn't provide such interface to ptracer to change vector > length. It does, with this patch, beacuse... > The vector length is sort of a read-only property of thread/process/ > program to debugger, unless we really have a clear requirement to modify > vector length in debugging. I may miss something because I haven't debug > SVE code yet. ...the vector length is no longer read-only for the task, thanks to the new prctls(). This does add complexity, but I figured that any programmer's model state that the thread can modify for itself should be modifiable by the debugger, if for no other reason than the user may want to experiment to see what happens. Without a ptrace interface, it would be necessary to inject a prctl() call into the target, which is possible but awkward. gdb must already re-detect the vector length on stop, since the target could have called the prctl() in the meantime. Access via ptrace also allows things like trapping on exec, fork or clone and changing the vector length for the new process or thread before it starts to run. I'm guessing here, but such a scenario seems legitimate (?) [...] Cheers ---Dave