From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="ne6OEVe2" Received: from mail-pf1-x44a.google.com (mail-pf1-x44a.google.com [IPv6:2607:f8b0:4864:20::44a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B13A210DB for ; Thu, 30 Nov 2023 08:09:29 -0800 (PST) Received: by mail-pf1-x44a.google.com with SMTP id d2e1a72fcca58-6cdf90d609aso129067b3a.0 for ; Thu, 30 Nov 2023 08:09:29 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1701360569; x=1701965369; 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=/RtjJzL/vc29lQ75YmuaxVjA9U45zH9PPJffTuX7MJ4=; b=ne6OEVe2j4B15Tu7606C8/CiOnlmaXQd6TRZip/t1f8sUlzOIOR2Q1FoCEmcA5MHv2 Nc+1LifGp0NV9K/jc27DHQ5K3VC3/h/zQ6b0qeiVEYdniBUMD/8b/9OlWrlBWLKBOmWI WO3U6WJlIUpueOBataf/wcxubZqcEyODevbNFrflbAR61ZBDqp9N+qNKKrzX67H9T2m1 baEyKuoLWBNRBI19vrARgZFMX2r1RA++wIUn/rT+dndbay9kd7RYgE86skgwIQMCt4N5 81EM2jqrcYU8mt7kbF7doVwWbIPfC2/ABQ/YpevlKaBw7PXH+Ks3ylj31PqmP1dQ3j0Y JYWA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1701360569; x=1701965369; 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=/RtjJzL/vc29lQ75YmuaxVjA9U45zH9PPJffTuX7MJ4=; b=ddYWlOqjgjOQH1VvSpVIyXtJAeZ1SrNkyWBgm2myuDSb4ozXgtpDtiJQFqD+HDntlb 3jKnhqL+rtXYsrnk6eCteW+A5TY43YUBhHo3P3iVc1YMWEugn8rpcl3atzEg+0uEMgHX ddNIKuYGLpiFsa+OcqXISTTdjHx/I5qBONZ7v+/wxdrSjAEtqUni5UTBaVSyxzRzsczf 93hC18qMQajjhBFCuwTodt9pXEHsAKNRWDymqL6Yx/uAevnORCByDfxjX4AzoOh/2pxl u/Mny0qSNTNnbz+u8vTPs+LaoHjYM5XY52wU4gfxx1rmKTU+Ip6ncufMoEQRE3SAB4Lo 8b6Q== X-Gm-Message-State: AOJu0YzCbwEOW08Zh/xI1vsfO1qBE5oHqkuk9jaJ+8lInL5TBtub8Uiy fyOd9OXVI6SHyMTUnvUDLlsw3PKdb7c= X-Google-Smtp-Source: AGHT+IH1QROfldGFep5xZUPIQJ+BAGxcr5i8FuC775Xv7pFv/SUbUB2OCG86SlGNVua4BY9Yx2bcQTt4Yzg= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:6a00:13a9:b0:6cd:acdf:5b0b with SMTP id t41-20020a056a0013a900b006cdacdf5b0bmr1979152pfg.6.1701360569282; Thu, 30 Nov 2023 08:09:29 -0800 (PST) Date: Thu, 30 Nov 2023 08:09:27 -0800 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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" On Thu, Nov 30, 2023, Xiaoyao Li wrote: > 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. Yes, -EIO is effectively ABI at this point. Though there is the caveat that -EIO doesn't guarantee KVM killed the VM, i.e. KVM could return -EIO for some other reason (though that's highly unlikely for KVM_RUN at least). > > 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. Ah. Not really. SHUTDOWN isn't fatal per se, e.g. QEMU emulates a RESET if a vCPU hits shutdown. INTERNAL_ERROR isn't always fatal on x86, e.g. QEMU ignores (I think that's what happens) emulation failure when the vCPU is at CPL > 0 so that guest userspace can't DoS the VM.