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 61D24C4708E for ; Wed, 4 Jan 2023 14:43:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239628AbjADOnM (ORCPT ); Wed, 4 Jan 2023 09:43:12 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33378 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239681AbjADOms (ORCPT ); Wed, 4 Jan 2023 09:42:48 -0500 Received: from mail-pl1-x634.google.com (mail-pl1-x634.google.com [IPv6:2607:f8b0:4864:20::634]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DD2191EACF for ; Wed, 4 Jan 2023 06:42:45 -0800 (PST) Received: by mail-pl1-x634.google.com with SMTP id d9so19310100pll.9 for ; Wed, 04 Jan 2023 06:42:45 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=XqrtrKRhDtPaDDzxdLxDCGFf7RuTdmXM7ZNOkYasJp0=; b=annFklxy87JJ3NRbnht+zPMvo+BAwdt75U4NiJoQVnyUOKlpPtp/cRuA2vofqUcQCi P0vuJl1DIw3GMfw5OTu1tZiv2Vj3qPg+WnN/Jx7K9J6RdGVZSi9zs+Zlp1gP2aqAhVjL AqzsIO9GT4hQEORUz+CRmEUeKbk0I42JGplO2YoztwiDlYiMb+TO9BJa/n3E5GXZhbuL GhjKblEIvWoseorJLshQnlvMK+QxSrk5smdrSblQmZ7FyzYfHXRZfElfeDV+lX1tb3YX bS2etoCnGfiK5NAvxGZ+DWDbLoy2IBD1CMYXN4G6CS+r9yhOO0i7f3uaOMNTgYA4dxv9 aDCw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=XqrtrKRhDtPaDDzxdLxDCGFf7RuTdmXM7ZNOkYasJp0=; b=h712TNhLjLpnmEj2GO1xBbrXtXWkFNLcSJUx+e6wRMOWfPOfBO75lx4kBV3RHb5hsE VXBH6ydB48YzNkg4qdFtlvZj0oXcPwB4kHXUldW2DXdob6rCtsUeU5O6MtvzuKGihnAY hiTJMYP5VJSJMvf75Oltge3RkipL+lVhq1pFwnWFP4/EtsMbywuTPGPwhb//kMujycT3 4tD9sYqtynrCZ0nWIspvbFjtv/i36ghzZ0NO7/m1QKmcYgPapR+P3v2N8nL6jnw1Hn1m LZnbuBEfHm13fgPz7CLOLuPJgKT0b8g768vQQdLqV/FTQy6OubW6RHb/kFfUcfRojxxO OH7g== X-Gm-Message-State: AFqh2kq4M3nl4uriy7cE514WWBlnHADwkilE7O3Z/3kSMUdNeCeM1yOF xxWz6B2WllAd4qG60n07shFNlw== X-Google-Smtp-Source: AMrXdXtZlSUzwJzX/CvX/YPyt/O1AXFajoGSeAYuzp0beESFSo5sq4QF1exhZz+z45WMIHfXVDWBFg== X-Received: by 2002:a05:6a20:7f59:b0:ac:af5c:2970 with SMTP id e25-20020a056a207f5900b000acaf5c2970mr3145986pzk.3.1672843364859; Wed, 04 Jan 2023 06:42:44 -0800 (PST) Received: from google.com (7.104.168.34.bc.googleusercontent.com. [34.168.104.7]) by smtp.gmail.com with ESMTPSA id z15-20020a63190f000000b0046ffe3fea77sm20377445pgl.76.2023.01.04.06.42.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Jan 2023 06:42:44 -0800 (PST) Date: Wed, 4 Jan 2023 14:42:40 +0000 From: Sean Christopherson To: Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Aaron Lewis , Yu Zhang Subject: Re: [PATCH v2 3/4] KVM: nVMX: Don't muck with allowed sec exec controls on CPUID changes Message-ID: References: <20221213062306.667649-1-seanjc@google.com> <20221213062306.667649-4-seanjc@google.com> <70872206-7a75-0a19-3df5-a97207e710fa@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 04, 2023, Sean Christopherson wrote: > On Fri, Dec 23, 2022, Paolo Bonzini wrote: > > On 12/13/22 07:23, Sean Christopherson wrote: > > > Don't modify the set of allowed secondary execution controls, i.e. the > > > virtual MSR_IA32_VMX_PROCBASED_CTLS2, in response to guest CPUID changes. > > > To avoid breaking old userspace that never sets the VMX MSRs, i.e. relies > > > on KVM to provide a consistent vCPU model, keep the existing behavior if > > > userspace has never written MSR_IA32_VMX_PROCBASED_CTLS2. > > > > > > KVM should not modify the VMX capabilities presented to L1 based on CPUID > > > as doing so may discard explicit settings provided by userspace. E.g. if > > > userspace does KVM_SET_MSRS => KVM_SET_CPUID and disables a feature in > > > the VMX MSRs but not CPUID (to prevent exposing the feature to L2), then > > > stuffing the VMX MSRs during KVM_SET_CPUID will expose the feature to L2 > > > against userspace's wishes. > > > > The commit message doesn't explain *why* KVM_SET_CPUID would be done before > > KVM_SET_MSRS. > > I assume you mean why KVM_SET_MSRS would be done before KVM_SET_CPUID2? > > This patch is mostly paranoia, AFAIK there is no userspace that is negatively > affected by KVM's manipulations. The only case I can think of is if userspace > wanted to emulate dynamic CPUID updates, e.g. set an MSR filter to intercept writes > to MISC_ENABLES to update MONITOR/MWAIT support, but that behavior isn't allowed > since commit feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN"). > > There are scenarios where userspace might do KVM_SET_MSRS before KVM_SET_CPUID, > e.g. QEMU's reuse of a vCPU for CPU hotplug, but in those cases I would expect > userspace to follow up with another KVM_SET_MSRS. An argument for taking this patch is that it might be necessary to disallow KVM_SET_MSRS after KVM_RUN[*]. If KVM manipulates VMX MSRs during KVM_SET_CPUID2, reusing a vCPU with sequence: SET_CPUID2 => SET_MSRS => RUN => unplug => hotplug => SET_CPUID2 => SET_MSRS sequence will cause the second SET_MSRS to fail due to userspace "changing" the MSR value. [*] https://lore.kernel.org/all/20220805172945.35412-4-seanjc@google.com