Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753445AbaJGMWX (ORCPT ); Tue, 7 Oct 2014 08:22:23 -0400 Received: from mailapp01.imgtec.com ([195.59.15.196]:28323 "EHLO mailapp01.imgtec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751334AbaJGMWW (ORCPT ); Tue, 7 Oct 2014 08:22:22 -0400 Message-ID: <5433DAFA.4010008@imgtec.com> Date: Tue, 7 Oct 2014 13:22:18 +0100 From: James Hogan User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: David Daney , "Kevin D. Kissell" , David Daney , , Leonid CC: , , David Daney Subject: Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area. References: <1412627010-4311-1-git-send-email-ddaney.cavm@gmail.com> <54333B9C.2040302@paralogos.com> <54336CED.3040106@gmail.com> <5433D429.3020804@imgtec.com> In-Reply-To: <5433D429.3020804@imgtec.com> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.154.101] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/10/14 12:53, James Hogan wrote: > On 07/10/14 05:32, David Daney wrote: >> If the kernel automatically allocated the emulation locations, what >> would happen if there were a signal that interrupted the emulation, and >> the signal handler did a longjump to somewhere else? How would we clean >> up the now unused emulation memory allocations? > > AFAICT, Leonid's implementation also has this problem, and that has a > separate stack of emuframes per thread managed completely by the kernel. > > Essentially the kernel doesn't manage the stack, userland does, and > userland can choose to skip over sigframes and emuframes with siglongjmp > without telling the kernel. > > Userland can even switch between contexts (which includes stack) with > setcontext (coroutines etc) which breaks the assumption in Leonid's > patches that emuframes will be completed in reverse order to them being > started, again demonstrating that it is essentially userland that > manages the stack. > > I think any attempt by the kernel to keep track of user stacks (e.g. by > storing a stack pointer along with the emuframe so that unused emuframes > can be discarded later when stack pointer goes high again) will be > foiled by setcontext. > > Hmm, I can't see a way forward that doesn't involve invasive userland > handling & ABI changes other than giving up with non-executable stacks > or limiting permitted instructions in delay slots to those Linux knows > how to emulate directly. Would it work for a signal encountered during branch delay slot emulation (maybe where the PC is pointing at that magic location the kernel uses for emulation) to be treated as a return from emulation, but leaving the user PC pointing to the original branch (with Cause.BD=1 I suppose) prior to handling the signal, so that no more than one emuframe is needed by each thread at a time? Cheers James -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/