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 BE462C4167B for ; Wed, 29 Nov 2023 19:23:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229509AbjK2TWK (ORCPT ); Wed, 29 Nov 2023 14:22:10 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50006 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231494AbjK2TWH (ORCPT ); Wed, 29 Nov 2023 14:22:07 -0500 Received: from mail-pg1-x549.google.com (mail-pg1-x549.google.com [IPv6:2607:f8b0:4864:20::549]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 39CE7D5E for ; Wed, 29 Nov 2023 11:22:12 -0800 (PST) Received: by mail-pg1-x549.google.com with SMTP id 41be03b00d2f7-5c5fe3b00f6so93756a12.2 for ; Wed, 29 Nov 2023 11:22:12 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1701285731; x=1701890531; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=zm+VmJgtsDhhnZtgTitQtOHEZp/dUiGbrU1wdemhaX4=; b=VvkVbyMZpBIEHXoL/pz/DHyw9r5iis0kPlBeiktc/qjUOAquMjPHnDDW0h3+A1HlId Qvg8IT6YTjCsMbAi/0jFCzTFE/Yav5PcHo4MketSk+ZVMrFHEO/d0GYwwlKgyXGCTo9V v/B/U8rnd80ng3HIeewZOyyk+bkB5eTUuCyKXWq6Rf9lu+m53FE8q5rLXxyjKXIFDo65 WHL1iDw1SgCveKuL8Tg7YPSvU1mf/5seObuRHPAnxxCumnj6NNX0WJac1I3VB4UrLkrB bbaEEUBDGdSlZECaiqZTlkhUlugC1jgUufGDEUVA1/cBCMw8SgocvYS22IijyKcT8Int RYfQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1701285731; x=1701890531; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=zm+VmJgtsDhhnZtgTitQtOHEZp/dUiGbrU1wdemhaX4=; b=YDoPIP7JMNFg5FuCkR7SNSsU0BgqPq8V5ymLrRWpmVp9JkgL+JqlUkiJYR7bjxOWHf vNAvNKyKlFi3QzVOpgXQxRaNFrwwNEFQt1CJuz8Cl/D/mXLTXLP7rJnlmrCcrXWJKMnc DJz7RcrX4QdlvkxnAmHI1r4qR4qsFkoWs2ahMkgTBIoKvmb9qw2Wz6bsCp7D2X9aLz/H OmKiFh41I2/Iyu275r4SSuB40am4yoX5a9xQnkN7x9u+mMhVih4EEfEeDa3qqg13ox3T 5UMwoMzUPC/ryI0zmAULK4IKxYb3AhELRnY5vM3oIzhllObH6mgL6DAhbNfPOA6JRduy Slqg== X-Gm-Message-State: AOJu0YzRuvw5BnMyie0lU5mjiju44VEOTJLzt36orcVCP8hM5pmL5zxl yKGp635cTtd+R6yhBFfotDLDRLmSArk= X-Google-Smtp-Source: AGHT+IFbnamXd5HIGOk9ycysykN0hM1eotS+NZRkIrasluqPuu9nCw73/x2uc9oduuUnuHto7tym2AJtkoA= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a63:ef09:0:b0:5be:71:f35d with SMTP id u9-20020a63ef09000000b005be0071f35dmr3359653pgh.1.1701285731587; Wed, 29 Nov 2023 11:22:11 -0800 (PST) Date: Wed, 29 Nov 2023 11:22:10 -0800 In-Reply-To: <22c602c9-4943-4a16-a12e-ffc5db29daa1@intel.com> Mime-Version: 1.0 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> Message-ID: Subject: Re: [PATCH v2 2/2] KVM: selftests: Add logic to detect if ioctl() failed because VM was killed From: Sean Christopherson To: Xiaoyao Li Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Michal Luczaj , Oliver Upton , Colton Lewis Content-Type: text/plain; charset="us-ascii" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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, 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.