From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751941AbeECMs6 (ORCPT ); Thu, 3 May 2018 08:48:58 -0400 Received: from mga06.intel.com ([134.134.136.31]:6049 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751845AbeECMsy (ORCPT ); Thu, 3 May 2018 08:48:54 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.49,358,1520924400"; d="scan'208";a="37159985" Date: Thu, 3 May 2018 15:48:47 +0300 From: Alexander Shishkin To: Paolo Bonzini Cc: Alexander Shishkin , Luwei Kang , kvm@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, rkrcmar@redhat.com, linux-kernel@vger.kernel.org, joro@8bytes.org, peterz@infradead.org, chao.p.peng@linux.intel.com Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace virtualization mode Message-ID: <20180503124847.tcrizs2sxy3j6mu2@um.fi.intel.com> References: <1525349323-9938-1-git-send-email-luwei.kang@intel.com> <1525349323-9938-7-git-send-email-luwei.kang@intel.com> <20180503113223.x2ykby6wglppgdwf@um.fi.intel.com> <47494e95-edcb-e8b5-0a32-004e9470dbdf@redhat.com> <20180503120215.dev7ti43zk7yoovn@um.fi.intel.com> <221c297b-1270-371b-70a3-4cff4bed7a7e@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <221c297b-1270-371b-70a3-4cff4bed7a7e@redhat.com> User-Agent: NeoMutt/20171215 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 03, 2018 at 02:30:25PM +0200, Paolo Bonzini wrote: > On 03/05/2018 14:02, Alexander Shishkin wrote: > > I don't see why. System-wide tracing takes place while perf record is > > running. When it's done, it's done and we can un-disable the guest > > tracing, without requiring the user to kill all their VMs and reload > > modules. > > Guest tracing can only be enabled at boot time, because the guest's > CPUID changes depending on whether it's enabled. And likewise if perf > record can do system-wide tracing at any time during the guest's > execution, we need to know it at boot time in order to set the guest CPUID. CPUID is immaterial here; the real trick is to disallow the use of PT at runtime when the host suddenly decides to trace the guest, in such a way that the guest user is informed that their trace is incomplete due to the host activity. Side note: the "system-wide tracing" is a misnomer here, all that matters is that there's a perf event on the host that wants to trace the guest, it can very well be a per-task event. Regards, -- Alex