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=-4.0 required=3.0 tests=BAYES_00,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 7C012C433E9 for ; Tue, 9 Mar 2021 13:44:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0F8BB64F71 for ; Tue, 9 Mar 2021 13:44:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231234AbhCINoB (ORCPT ); Tue, 9 Mar 2021 08:44:01 -0500 Received: from mail.kernel.org ([198.145.29.99]:53188 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231137AbhCINnn (ORCPT ); Tue, 9 Mar 2021 08:43:43 -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 0BEDF64F51; Tue, 9 Mar 2021 13:43:43 +0000 (UTC) Received: from 78.163-31-62.static.virginmediabusiness.co.uk ([62.31.163.78] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94) (envelope-from ) id 1lJceC-000YiA-T1; Tue, 09 Mar 2021 13:43:41 +0000 Date: Tue, 09 Mar 2021 13:43:40 +0000 Message-ID: <87mtvcxx0z.wl-maz@kernel.org> From: Marc Zyngier To: Andrew Jones Cc: kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, James Morse , Julien Thierry , Suzuki K Poulose , Alexandru Elisei , kernel-team@android.com Subject: Re: [PATCH] KVM: arm64: Cap default IPA size to the host's own size In-Reply-To: <20210309132021.7vuuf73joybhlhg3@kamzik.brq.redhat.com> References: <20210308174643.761100-1-maz@kernel.org> <20210309132021.7vuuf73joybhlhg3@kamzik.brq.redhat.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 62.31.163.78 X-SA-Exim-Rcpt-To: drjones@redhat.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, alexandru.elisei@arm.com, kernel-team@android.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Hi Andrew, On Tue, 09 Mar 2021 13:20:21 +0000, Andrew Jones wrote: > > Hi Marc, > > On Mon, Mar 08, 2021 at 05:46:43PM +0000, Marc Zyngier wrote: > > KVM/arm64 has forever used a 40bit default IPA space, partially > > due to its 32bit heritage (where the only choice is 40bit). > > > > However, there are implementations in the wild that have a *cough* > > much smaller *cough* IPA space, which leads to a misprogramming of > > VTCR_EL2, and a guest that is stuck on its first memory access > > if userspace dares to ask for the default IPA setting (which most > > VMMs do). > > > > Instead, cap the default IPA size to what the host can actually > > do, and spit out a one-off message on the console. The boot warning > > is turned into a more meaningfull message, and the new behaviour > > is also documented. > > > > Although this is a userspace ABI change, it doesn't really change > > much for userspace: > > > > - the guest couldn't run before this change, while it now has > > a chance to if the memory range fits the reduced IPA space > > > > - a memory slot that was accepted because it did fit the default > > IPA space but didn't fit the HW constraints is now properly > > rejected > > I'm not sure deferring the misconfiguration error until memslot > request time is better than just failing to create a VM. If > userspace doesn't use KVM_CAP_ARM_VM_IPA_SIZE to determine the > limit (which it hasn't been obliged to do) and it is able to > successfully create a VM, then it will assume up to 40-bit IPAs > are supported. Later, when it tries to add memslots and fails > it may be confused, especially if that later is much, much later > with memory hotplug. That's a fair point. However, no existing userspace will work on these systems. Is that what we want to do? I don't care much, but having non-usable defaults feel a bit... odd. I do spit out a warning, but I agree this isn't great either. > > The other thing that's left doing is to convince userspace to > > actually use the IPA space setting instead of relying on the > > antiquated default. > > Failing to create any VM which hasn't selected a valid IPA limit > should be pretty convincing :-) I'll make sure to redirect the reports your way! :D Thanks, M. -- Without deviation from the norm, progress is not possible.