From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eduardo Habkost Subject: Re: [PATCH RESEND v1 1/2] i386: Add Intel Processor Trace feature support Date: Mon, 15 Jan 2018 12:04:55 -0200 Message-ID: <20180115140455.GN6646@localhost.localdomain> References: <1515443797-10837-1-git-send-email-luwei.kang@intel.com> <20180112142252.GG6646@localhost.localdomain> <82D7661F83C1A047AF7DC287873BF1E167E7630B@SHSMSX101.ccr.corp.intel.com> <97bf8d29-4009-6f61-f4bb-d128da5a4c9f@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Kang, Luwei" , "qemu-devel@nongnu.org" , "kvm@vger.kernel.org" , "rth@twiddle.net" , "mtosatti@redhat.com" , Chao Peng , Jiri Denemark , libvir-list@redhat.com To: Paolo Bonzini Return-path: Received: from mx1.redhat.com ([209.132.183.28]:51530 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965884AbeAOOFJ (ORCPT ); Mon, 15 Jan 2018 09:05:09 -0500 Content-Disposition: inline In-Reply-To: <97bf8d29-4009-6f61-f4bb-d128da5a4c9f@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: CCing libvirt developers. On Mon, Jan 15, 2018 at 10:33:35AM +0100, Paolo Bonzini wrote: > On 15/01/2018 08:19, Kang, Luwei wrote: > >> If you are forwarding host info directly to the guest, the feature > >> is not migration-safe. The new feature needs to be added to > >> feature_word_info[FEAT_7_0_EBX].unmigratable_flags. > >> > > Hi, Thanks for you review. I want to support Intel PT live migration > > and patch [2/2] to do this. I don't understand why need to add this > > feature in feature_word_info[FEAT_7_0_EBX].unmigratable_flags first > > to disable live migration. > > Hi Luwei, > > the issue is that different hosts can have different CPUID flags. You > don't have a way to set the CPUID flags from the "-cpu" command line, > and it's not clear what values will be there in the various Ice Lake SKUs. > > I am not sure if there is a mechanism to allow live migration only for > "-cpu host", but it cannot be supported for any other "-cpu" model. IIRC, we don't have any mechanism to actually prevent migration if an unmigratable flag is enabled. We just avoid enabling them by accident on "-cpu host". This case is slightly more problematic, however: the new feature is actually migratable (under very controlled circumstances) because of patch 2/2, but it is not migration-safe[1]. This means libvirt shouldn't include it in "host-model" expansion (which uses the query-cpu-model-expansion QMP command) until we make the feature migration-safe. For QEMU, this means the feature shouldn't be returned by "query-cpu-model-expansion type=static model=max" (but it can be returned by "query-cpu-model-expansion type=full model=max"). Jiri, it looks like libvirt uses type=full on query-cpu-model-expansion on x86. It needs to use type=static[2], or it will have no way to find out if a feature is migration-safe or not. This case is similar to "pmu", which is not migration-safe but enabled by -cpu host by default. But "pmu" is less problematic because it is already skipped by query-cpu-model-expansion type=static. --- [1] "migration-safe" is defined in the documentation for CpuDefinitionInfo on the QAPI schema: # @migration-safe: whether a CPU definition can be safely used for # migration in combination with a QEMU compatibility machine # when migrating between different QMU versions and between # hosts with different sets of (hardware or software) # capabilities. If not provided, information is not available # and callers should not assume the CPU definition to be # migration-safe. (since 2.8) [2] It looks like libvirt uses type=full because it wants to get all QOM property aliases returned. In this case, one solution for libvirt is to use: static_expansion = query_cpu_model_expansion(type=static, model) all_props = query_cpu_model_expansion(type=full, static_expansion) -- Eduardo