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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 D26BEC2BA80 for ; Mon, 6 Apr 2020 21:32:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B3CD3206C3 for ; Mon, 6 Apr 2020 21:32:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726331AbgDFVci (ORCPT ); Mon, 6 Apr 2020 17:32:38 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:45743 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725895AbgDFVch (ORCPT ); Mon, 6 Apr 2020 17:32:37 -0400 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jLZM7-0001C1-U3; Mon, 06 Apr 2020 23:32:32 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 5FC34100C47; Mon, 6 Apr 2020 23:32:31 +0200 (CEST) From: Thomas Gleixner To: Vivek Goyal , Peter Zijlstra Cc: Andy Lutomirski , Paolo Bonzini , LKML , X86 ML , kvm list , stable Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS In-Reply-To: <20200406190951.GA19259@redhat.com> References: <87ftek9ngq.fsf@nanos.tec.linutronix.de> <87a74s9ehb.fsf@nanos.tec.linutronix.de> <87wo7v8g4j.fsf@nanos.tec.linutronix.de> <877dzu8178.fsf@nanos.tec.linutronix.de> <37440ade-1657-648b-bf72-2b8ca4ac21ce@redhat.com> <871rq199oz.fsf@nanos.tec.linutronix.de> <87d09l73ip.fsf@nanos.tec.linutronix.de> <20200309202215.GM12561@hirez.programming.kicks-ass.net> <20200406190951.GA19259@redhat.com> Date: Mon, 06 Apr 2020 23:32:31 +0200 Message-ID: <87wo6sjo5s.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Vivek Goyal writes: > On Mon, Mar 09, 2020 at 09:22:15PM +0100, Peter Zijlstra wrote: >> On Mon, Mar 09, 2020 at 08:05:18PM +0100, Thomas Gleixner wrote: >> > Andy Lutomirski writes: >> >> > > I'm okay with the save/restore dance, I guess. It's just yet more >> > > entry crud to deal with architecture nastiness, except that this >> > > nastiness is 100% software and isn't Intel/AMD's fault. >> > >> > And we can do it in C and don't have to fiddle with it in the ASM >> > maze. >> >> Right; I'd still love to kill KVM_ASYNC_PF_SEND_ALWAYS though, even if >> we do the save/restore in do_nmi(). That is some wild brain melt. Also, >> AFAIK none of the distros are actually shipping a PREEMPT=y kernel >> anyway, so killing it shouldn't matter much. > > It will be nice if we can retain KVM_ASYNC_PF_SEND_ALWAYS. I have another > use case outside CONFIG_PREEMPT. > > I am trying to extend async pf interface to also report page fault errors > to the guest. > > https://lore.kernel.org/kvm/20200331194011.24834-1-vgoyal@redhat.com/ > > Right now async page fault interface assumes that host will always be > able to successfully resolve the page fault and sooner or later PAGE_READY > event will be sent to guest. And there is no mechnaism to report the > errors back to guest. > > I am trying to add enhance virtiofs to directly map host page cache in guest. > > https://lore.kernel.org/linux-fsdevel/20200304165845.3081-1-vgoyal@redhat.com/ > > There it is possible that a file page on host is mapped in guest and file > got truncated and page is not there anymore. Guest tries to access it, > and it generates async page fault. On host we will get -EFAULT and I > need to propagate it back to guest so that guest can either send SIGBUS > to process which caused this. Or if kernel was trying to do memcpy(), > then be able to use execpetion table error handling and be able to > return with error. (memcpy_mcflush()). > > For the second case to work, I will need async pf events to come in > even if guest is in kernel and CONFIG_PREEMPT=n. What? > So it would be nice if we can keep KVM_ASYNC_PF_SEND_ALWAYS around. No. If you want this stuff to be actually useful and correct, then please redesign it from scratch w/o abusing #PF. It want's to be a separate vector and then the pagefault resulting from your example above becomes a real #PF without any bells and whistels. Thanks, tglx