From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751403AbeECNie (ORCPT ); Thu, 3 May 2018 09:38:34 -0400 Received: from mga03.intel.com ([134.134.136.65]:52117 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751120AbeECNia (ORCPT ); Thu, 3 May 2018 09:38:30 -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="48014210" Date: Thu, 3 May 2018 16:38:23 +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: <20180503133823.tqbraql3b7ltuo6a@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> <20180503124847.tcrizs2sxy3j6mu2@um.fi.intel.com> <26505f7a-9e6b-efb9-e5f9-8aae3756be49@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <26505f7a-9e6b-efb9-e5f9-8aae3756be49@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 02:50:12PM +0200, Paolo Bonzini wrote: > On 03/05/2018 14:48, Alexander Shishkin wrote: > >> 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. > > How do you do that? Off the top of my head: * you don't; * you write something to the PT stream; * you signal an error via RTIT_STATUS; * guest always prevails: host gets PARTIAL records in case of a conflict. > And you still need the module parameter to decide > whether the host is _allowed_ to cause incomplete traces in the guest. Or rather a parameter to decide who wins in case both host and guest want to trace the guest. That's arguably better than having different versions of PT in the guest depending on a module parameter setting. Regards, -- Alex