From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v7 06/13] KVM: x86: Add Intel Processor Trace virtualization mode Date: Thu, 3 May 2018 15:48:12 +0200 Message-ID: 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> <20180503133823.tqbraql3b7ltuo6a@um.fi.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: 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 To: Alexander Shishkin Return-path: In-Reply-To: <20180503133823.tqbraql3b7ltuo6a@um.fi.intel.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 03/05/2018 15:38, Alexander Shishkin wrote: > 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. It's not different versions; it's having PT vs. not having PT at all. I don't really see it as a big issue. The nice thing about this series is that the interactions between PT code and KVM code are minimal. Paolo