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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5B533C4167B for ; Thu, 30 Nov 2023 03:07:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344162AbjK3DEK (ORCPT ); Wed, 29 Nov 2023 22:04:10 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33320 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230393AbjK3DEJ (ORCPT ); Wed, 29 Nov 2023 22:04:09 -0500 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.12]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F1589D6C; Wed, 29 Nov 2023 19:04:13 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1701313455; x=1732849455; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=GCiIaCw+VRboLufR/lMo0OoCNHsxCHgA0wMyBlShb1o=; b=SQwNECuH1JCgOtntM9c3ytdKWjE+PxAzqLVvOaBfUVlssD49d2hUo8+3 EiEKnIDIB8yfL8nx2Lnk+u2z2jBBeyrpmm6eS0877gHDK0bbf099XlGYK VFwMpRrJpWxtfH/UtbBFL5whKl2GwDWASiIOWNt0+OiEOayWh67q6yN1u 6tofHGzJYqsjMUqDJDm7l2NAacxlRRZOTa1R6ezPzumX+rVmOweDcUeoc ve8pFGecz4wGHHgGoOsYytJHV6TPKDBQmiypgTuY0M7G8Zo6e3SmKBvZX UM06DYaa4MJhzk9LfxK7PQ1fOKiDWEZxEfPfmTTVEWkItgi6lEKfmSBMT g==; X-IronPort-AV: E=McAfee;i="6600,9927,10909"; a="245356" X-IronPort-AV: E=Sophos;i="6.04,237,1695711600"; d="scan'208";a="245356" Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by orvoesa104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Nov 2023 19:04:13 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10909"; a="1100767184" X-IronPort-AV: E=Sophos;i="6.04,237,1695711600"; d="scan'208";a="1100767184" Received: from xiaoyaol-hp-g830.ccr.corp.intel.com (HELO [10.93.29.154]) ([10.93.29.154]) by fmsmga005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Nov 2023 19:04:10 -0800 Message-ID: Date: Thu, 30 Nov 2023 11:04:08 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] KVM: selftests: Add logic to detect if ioctl() failed because VM was killed To: Sean Christopherson Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Michal Luczaj , Oliver Upton , Colton Lewis References: <20231108010953.560824-1-seanjc@google.com> <20231108010953.560824-3-seanjc@google.com> <0ee32216-e285-406f-b20d-dd193b791d2b@intel.com> <22c602c9-4943-4a16-a12e-ffc5db29daa1@intel.com> Content-Language: en-US From: Xiaoyao Li In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/30/2023 3:22 AM, Sean Christopherson wrote: > On Mon, Nov 13, 2023, Xiaoyao Li wrote: >> On 11/9/2023 12:07 AM, Sean Christopherson wrote: >>> On Wed, Nov 08, 2023, Xiaoyao Li wrote: >>>> On 11/8/2023 9:09 AM, Sean Christopherson wrote: >>>>> Add yet another macro to the VM/vCPU ioctl() framework to detect when an >>>>> ioctl() failed because KVM killed/bugged the VM, i.e. when there was >>>>> nothing wrong with the ioctl() itself. If KVM kills a VM, e.g. by way of >>>>> a failed KVM_BUG_ON(), all subsequent VM and vCPU ioctl()s will fail with >>>>> -EIO, which can be quite misleading and ultimately waste user/developer >>>>> time. >>>>> >>>>> Use KVM_CHECK_EXTENSION on KVM_CAP_USER_MEMORY to detect if the VM is >>>>> dead and/or bug, as KVM doesn't provide a dedicated ioctl(). Using a >>>>> heuristic is obviously less than ideal, but practically speaking the logic >>>>> is bulletproof barring a KVM change, and any such change would arguably >>>>> break userspace, e.g. if KVM returns something other than -EIO. >>>> >>>> We hit similar issue when testing TDX VMs. Most failure of SEMCALL is >>>> handled with a KVM_BUG_ON(), which leads to vm dead. Then the following >>>> IOCTL from userspace (QEMU) and gets -EIO. >>>> >>>> Can we return a new KVM_EXIT_VM_DEAD on KVM_REQ_VM_DEAD? >>> >>> Why? Even if KVM_EXIT_VM_DEAD somehow provided enough information to be useful >>> from an automation perspective, the VM is obviously dead. I don't see how the >>> VMM can do anything but log the error and tear down the VM. KVM_BUG_ON() comes >>> with a WARN, which will be far more helpful for a human debugger, e.g. because >>> all vCPUs would exit with KVM_EXIT_VM_DEAD, it wouldn't even identify which vCPU >>> initially triggered the issue. >> >> It's not about providing more helpful debugging info, but to provide a >> dedicated notification for VMM that "the VM is dead, all the following >> command may not response". With it, VMM can get rid of the tricky detection >> like this patch. > > But a VMM doesn't need this tricky detection, because this tricky detections isn't > about detecting that the VM is dead, it's all about helping a human debug why a > test failed. > > -EIO already effectively says "the VM is dead", e.g. QEMU isn't going to keep trying > to run vCPUs. If -EIO for KVM ioctl denotes "the VM is dead" is to be the officially announced API, I'm fine. > Similarly, selftests assert either way, the goal is purely to print > out a unique error message to minimize the chances of confusing the human running > the test (or looking at results). > >>> Definitely a "no" on this one. As has been established by the guest_memfd series, >>> it's ok to return -1/errno with a valid exit_reason. >>> >>>> But I'm wondering if any userspace relies on -EIO behavior for VM DEAD case. >>> >>> I doubt userspace relies on -EIO, but userpsace definitely relies on -1/errno being >>> returned when a fatal error. >> >> what about KVM_EXIT_SHUTDOWN? Or KVM_EXIT_INTERNAL_ERROR? > > I don't follow, I was trying to ask if KVM_EXIT_SHUTDOWN and KVM_EXIT_INTERNAL_ERROR are treated as fatal error by userspace. > those are vcpu_run.exit_reason values, not errno values. Returning > any flavor of KVM_EXIT_*, which are positive values, would break userspace, e.g. > QEMU explicitly looks for "ret < 0", and glibc only treats small-ish negative > values as errors, i.e. a postive return value will be propagated verbatim up to > QEMU.