From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Morse Subject: Re: [PATCH v4 09/21] KVM: arm/arm64: mask/unmask daif around VHE guests Date: Fri, 03 Nov 2017 17:19:40 +0000 Message-ID: <59FCA52C.1070406@arm.com> References: <20171019145807.23251-1-james.morse@arm.com> <20171019145807.23251-10-james.morse@arm.com> <20171030074019.GS2166@lvm> <59FB0C24.7080503@arm.com> <20171103124502.GB14144@cbox> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 6CA1049D32 for ; Fri, 3 Nov 2017 13:19:44 -0400 (EDT) Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id kVBfCOtj9+B6 for ; Fri, 3 Nov 2017 13:19:43 -0400 (EDT) Received: from foss.arm.com (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by mm01.cs.columbia.edu (Postfix) with ESMTP id EFD0140D25 for ; Fri, 3 Nov 2017 13:19:42 -0400 (EDT) In-Reply-To: <20171103124502.GB14144@cbox> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu To: Christoffer Dall Cc: Jonathan.Zhang@cavium.com, Marc Zyngier , Catalin Marinas , Julien Thierry , Will Deacon , wangxiongfeng2@huawei.com, linux-arm-kernel@lists.infradead.org, Dongjiu Geng , kvmarm@lists.cs.columbia.edu List-Id: kvmarm@lists.cs.columbia.edu Hi Christoffer, On 03/11/17 12:45, Christoffer Dall wrote: > On Thu, Nov 02, 2017 at 12:14:28PM +0000, James Morse wrote: >> On 30/10/17 07:40, Christoffer Dall wrote: >>> On Thu, Oct 19, 2017 at 03:57:55PM +0100, James Morse wrote: >>>> Non-VHE systems take an exception to EL2 in order to world-switch into the >>>> guest. When returning from the guest KVM implicitly restores the DAIF >>>> flags when it returns to the kernel at EL1. >>>> >>>> With VHE none of this exception-level jumping happens, so KVMs >>>> world-switch code is exposed to the host kernel's DAIF values, and KVM >>>> spills the guest-exit DAIF values back into the host kernel. >>>> On entry to a guest we have Debug and SError exceptions unmasked, KVM >>>> has switched VBAR but isn't prepared to handle these. On guest exit >>>> Debug exceptions are left disabled once we return to the host and will >>>> stay this way until we enter user space. >>>> Give me a kick if you want this reworked as a fix (which will then >>>> conflict with this series), or a backportable version. >>> >>> I don't know of any real-world issues where some more graceful handling >>> of SErrors would make sense on older kernels, so I'm fine with just >>> merging this together with this series. >> >> What about debug? > Are we unmasking debug exceptions as we should with this patch? With this patch, yes, it directly restores the DAIF flags the arch code wants for irq-masked process-context. Debug is re-enabled. > If so, I suppose that could be required for something like kgdb or when > running KVM as a guest hypervisor (nested). > > In that case, we should probably provide a backport for stable, if we > think people are going to be running older kernels on VHE systems, which > they probably are. Okay, I will produce a backport once this gets merged. >>> On guest exit Debug exceptions are left disabled once we return to the host >>> and will stay this way until we enter user space. > [The indentation seems to indicate I wrote this, but I don't think I > did. I'm confused.] I quoted it from the commit message, but evidently not from this depth-of-reply. Sorry for the confusion. >> Today VHE:KVM causes the kernel to run with SError unmasked and debug disabled >> until the next return to user-space, whereas previously the kernel expected >> SError to be masked and debug enabled. >> >> >> (Reposting just the SError rework without this patch changes the kernel to >> expect SError to be unmasked, which isn't making this any worse.) > I'm sorry, I don't understand this discussion. What is today, and what English has failed me. I'll try again: v4.14-rc7 with VHE causes the kernel to run after guest-exit with SError unmasked and debug disabled until the next return to user-space. The arch code expects SError masked and debug enabled. In your kgdb example, if we switch-to a new task instead of returning to user space, it won't hit any break/watchpoints. > is previously, and are you suggesting we drop this patch, or that the > rest of this series is somehow going to be applied without this patch? I reposted just the SError rework, patches 1-10 without this patch. If merged, this would change the arch code to expect SError to be unmasked from process context, leaving just the debug disabled after VHE guest-exit. I was (hurriedly) trying to work out if reposting the SError-rework without this patch made the situation worse. Sorry for the confusion! James > Reset: I think this patch is fine in the context of this series.. I now > have no idea what we need to do in terms of older kernels. > > Thanks, > -Christoffer >