Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Alexander Shishkin <alexander.shishkin@linux.intel.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Luwei Kang <luwei.kang@intel.com>, kvm@vger.kernel.org,
        x86@kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        rkrcmar@redhat.com, joro@8bytes.org, songliubraving@fb.com,
        peterz@infradead.org, kstewart@linuxfoundation.org,
        gregkh@linuxfoundation.org, thomas.lendacky@amd.com,
        konrad.wilk@oracle.com, mattst88@gmail.com,
        Janakarajan.Natarajan@amd.com, dwmw@amazon.co.uk,
        jpoimboe@redhat.com, marcorr@google.com, ubizjak@gmail.com,
        sean.j.christopherson@intel.com, jmattson@google.com,
        linux-kernel@vger.kernel.org,
        Chao Peng <chao.p.peng@linux.intel.com>
Subject: Re: [PATCH v13 08/12] KVM: x86: Add Intel PT context switch for each vcpu
In-Reply-To: <b694312e-b668-3546-f9e8-dc740f330f14@redhat.com>
References: <1540368316-12998-1-git-send-email-luwei.kang@intel.com> <1540368316-12998-9-git-send-email-luwei.kang@intel.com> <87a7n37iuf.fsf@ashishki-desk.ger.corp.intel.com> <bc192976-2c56-3cfc-a697-cf941a92ad0a@redhat.com> <alpine.DEB.2.21.1810301058060.5984@nanos.tec.linutronix.de> <b694312e-b668-3546-f9e8-dc740f330f14@redhat.com>
Date:   Wed, 31 Oct 2018 13:46:24 +0200
Message-ID: <87o9ba5ofj.fsf@ashishki-desk.ger.corp.intel.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 30/10/2018 11:00, Thomas Gleixner wrote:
>> So at least we need a way for perf on the host to programmatically detect,
>> that 'guest traces itself' is enabled, so it can inject that information
>> into the host data and post processing can tell that. W/o something like
>> that it's going to be a FAQ.
>
> In guest-tracing mode there will be already a TIP.PGD and TIP.PGE packet
> respectively before vmentry and after vmexit, caused by the RTIT_CTL
> WRMSRs in pt_guest_enter and pt_guest_exit.  The target IP of the
> packets will come from kvm-intel.ko.

Most people aren't tracing the kernel, so they'd just get a PGD with no
address and a PGE after the kvm is done without any indication of what
happened in between.

> In system mode instead you get a Paging Information Packet on
> vmentry/vmexit, with bit 0 set in the third byte.  You won't get it if
> guest-side tracing is on (because tracing has been disabled by
> pt_guest_enter and won't be re-enabled until pt_guest_exit).  I don't
> think it's correct to "fake" the PIP in guest-tracing mode, because
> TIP.PGD should be followed immediately by TIP.PGE.

Indeed, we should most definitely not fake PIP. Perf has RECORD_AUX,
which already has PARTIAL flag that was introduced specifically because
of kvm.

Regards,
--
Alex