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[46.102.197.194]) by smtp.gmail.com with ESMTPSA id l19-20020a05600c4f1300b0040772138bb7sm2489965wmq.2.2023.11.09.07.48.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Nov 2023 07:48:03 -0800 (PST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 5ee86532-7f17-11ee-9b0e-b553b5be7939 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cloud.com; s=cloud; t=1699544884; x=1700149684; darn=lists.xenproject.org; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:subject:cc:to:from:date:message-id:from:to :cc:subject:date:message-id:reply-to; bh=GdGoKsvszk1er7lcMQ58LvUTm+ralEqFHg2PA8LdxCE=; b=DrFVb5aoAboCBFLF8xjuCWm6A+dDf9evboa1vYsSuAc33/PxJijv75SV6sziEHDLte smNQUq7051lRVwf02HPY7kr2lu+p4nIJw4dI/2hHHwDjWk7dqVJ8JBz/GyKqZJoPP0S6 RQ2qsj1yXLdSprD19ZcNtMPpxroADpQsxyvIY= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1699544884; x=1700149684; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:subject:cc:to:from:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=GdGoKsvszk1er7lcMQ58LvUTm+ralEqFHg2PA8LdxCE=; b=nsm6LX9BJQbvK7gbi1plbJDb/4jlsNnwOovzJCLNtGT5ZGLPQnJ6bkr67VTWAyY2JH 5RzILOuEA2ZNvmFmbHzwgKOXCzJDQQU2KRuqXlJw9r3aQ47P7E1ZLJIZ28mLMbI4gKpo 5OH3TWYejFymmP6eSIoO55EA2+P97nZZEbtBTM0Mz2dKBFNKefgbEB7WSHmWf5z7D2kj ftzxF56wKYUkt6V/yOMtmEnMxRkoLNOKaz/eLS7adMXtXK16xG+1YanKDP6y541g1cEj 4nwYUBV+7uEXYeAWuisAJk1z6UcUzW3Rv0xNRF7f6qha6npxG+V5ZFObFKw420wsGzqB 1vxQ== X-Gm-Message-State: AOJu0YwN81J9+wrcWFVfF63W+GV96/y3XKNla8IzExYgiNnQQPIT4Yyw FevdUbYmEn+g9gtS+q57mPuhuQ== X-Google-Smtp-Source: AGHT+IGKeiI8aPgI1sHfiMXh9ybACJTRaFcogZXVx7uNvt2A89SU0kS4Eh5CrBoMjh70Xw3+vJP0AQ== X-Received: by 2002:a19:8c19:0:b0:4ff:9a75:211e with SMTP id o25-20020a198c19000000b004ff9a75211emr1692608lfd.42.1699544884364; Thu, 09 Nov 2023 07:48:04 -0800 (PST) Message-ID: <654cff33.050a0220.d1942.a8f1@mx.google.com> X-Google-Original-Message-ID: Date: Thu, 9 Nov 2023 15:48:00 +0000 From: Alejandro Vallejo To: Andrew Cooper Cc: Elliott Mitchell , xen-devel@lists.xenproject.org Subject: Re: Support situation for nestedhvm References: <93db0a9e-1d99-4953-9e3f-7ad69f0e78bf@tibco.com> <654cab6f.050a0220.86158.519b@mx.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Nov 09, 2023 at 10:36:21AM +0000, Andrew Cooper wrote: > On 09/11/2023 9:50 am, Alejandro Vallejo wrote: > > Hi, > > > > On Tue, Nov 07, 2023 at 08:15:32PM +0000, Andrew Cooper wrote: > >> On 07/11/2023 7:53 pm, Elliott Mitchell wrote: > >>> I ran into the nestedhvm via the following path. I was considering the > >>> feasibility of shedding tasks from a desktop onto a server running Xen. > >>> I was looking at `man xl.cfg` and noticed "nestedhvm". > >>> > >>> Since one of the tasks the computer handled was running other OSes in > >>> fully simulated environments, this seemed to be something I was looking > >>> for. No where did I ever see anything hinting "This configuration option > >>> is completely unsupported and risky to use". > >> This one is explicitly covered in SUPPORT.md, and has had XSAs out > >> against it in the past for being unexpectedly active when it oughtn't to > >> have been. > >> > >>> Things simply started exploding without any warnings. > >> Things also explode if you try to create a VM with 10x more RAM than you > >> have, or if you try `./xenwatchdogd --help`, or `xl debug-keys c`, or > >> many other things.  > >> > >> The xl manpage probably ought to state explicitly that the option is > >> experimental, but that the extent of what I'd consider reasonable here. > >> > >> You can't solve educational matters with technical measures. > >> > >> ~Andrew > >> > > No, but we can prevent users unexpectedly shooting themselves in the foot. > > ... and break OSSTest and XenRT while you're at it. > > Like it or not, this knob is behaved in this way for 15 years.  You will > be doing harm for no benefit by trying to change it. Improving UX is a distinctively good benefit. A lot of people on this mailing list may be aware of its quirks, but a user shouldn't need to be that aware in order to set up a stable system. > > And if you need a cautionary tail on why this is a bad idea generally, > as well as a background on why I will firmly object to technical > countermeasures like this, read up on Xen's allow_unsafe command line > parameter. > > ~Andrew This? https://bugzilla.redhat.com/show_bug.cgi?id=858724 If so, that's very different. allow_unsafe caused previously accesible remote hosts to become unbootable after an update, leaving anyone with a remote host without IPMI interface dead in the water. It's nothing like preventing spinning up a VM with a set of features that with high likelihood a user doesn't want. Both OSSTest and XenRT can simply adjust their nestedhvm knobs based on a simple probing script. Cheers, Alejandro