From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 365ABC433F5 for ; Mon, 25 Apr 2022 13:00:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238496AbiDYNDd (ORCPT ); Mon, 25 Apr 2022 09:03:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45830 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231466AbiDYNDb (ORCPT ); Mon, 25 Apr 2022 09:03:31 -0400 Received: from mail.efficios.com (mail.efficios.com [167.114.26.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9377F1025 for ; Mon, 25 Apr 2022 06:00:25 -0700 (PDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id BC78D3CDE07; Mon, 25 Apr 2022 09:00:23 -0400 (EDT) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id jR4M7k6F6weu; Mon, 25 Apr 2022 09:00:23 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 223B13CD978; Mon, 25 Apr 2022 09:00:23 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com 223B13CD978 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1650891623; bh=yHm+2z0bhIW3H5b+FeLjj8k5d02IRyDscMJqqe+OG6I=; h=Date:From:To:Message-ID:MIME-Version; b=tVTd2v8u8dXCrdmlhhj338WVIxh4hBL55YU+UO/elpbbsK7BIfKFQXwER9xdHhhGC BqU91HAVWdXovy5x5850oU3Rs97v4kzMkxlvcF743FbC2tpCWlC6HZiG+NgvHgWwlO NtfRHcJ7t1t2ER8xKiEfVs6if7aoHQnt2wayCHK1XIXkcgPn5HLSM3Eo8AUVFv9zNI gZXGbEAWSqGHe+tTyCTqmM1rQZnVJtDYx4fMnBCZJs51MWymtna8lRqOgR+qmgscnW GVP1pO/chtPKHM+04Yxp4xH6Uqoqa6hm7qxOcE1D98DXxw2Jh+k0NiW24ynaVdP2hJ ZRcR85vsC4Wfg== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id 4A6KmXiIth6A; Mon, 25 Apr 2022 09:00:23 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id 10C3C3CD977; Mon, 25 Apr 2022 09:00:23 -0400 (EDT) Date: Mon, 25 Apr 2022 09:00:22 -0400 (EDT) From: Mathieu Desnoyers To: Paolo Bonzini Cc: Sean Christopherson , lttng-dev , rostedt , KVM list , Mathieu Desnoyers Message-ID: <358746537.34457.1650891622938.JavaMail.zimbra@efficios.com> In-Reply-To: <1622857974.11247.1649441213797.JavaMail.zimbra@efficios.com> References: <1218866473.10909.1649432186473.JavaMail.zimbra@efficios.com> <3c11308e-006a-a7e9-8482-c6b341690530@redhat.com> <1622857974.11247.1649441213797.JavaMail.zimbra@efficios.com> Subject: Re: [lttng-dev] Unexport of kvm_x86_ops vs tracer modules MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_4257 (ZimbraWebClient - FF99 (Linux)/8.8.15_GA_4257) Thread-Topic: Unexport of kvm_x86_ops vs tracer modules Thread-Index: HRgc0Gw3F7w+0GLEUzyCTgfg1qIAWyO4TBSN Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org ----- On Apr 8, 2022, at 2:06 PM, Mathieu Desnoyers via lttng-dev lttng-dev@lists.lttng.org wrote: > ----- On Apr 8, 2022, at 12:24 PM, Paolo Bonzini pbonzini@redhat.com wrote: > >> On 4/8/22 17:36, Mathieu Desnoyers wrote: >>> LTTng is an out of tree kernel module, which currently relies on the export. >>> Indeed, arch/x86/kvm/x86.c exports a set of tracepoints to kernel modules, e.g.: >>> >>> EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_entry) >>> >>> But any probe implementation hooking on that tracepoint would need kvm_x86_ops >>> to translate the struct kvm_vcpu * into meaningful tracing data. >>> >>> I could work-around this on my side in ugly ways, but I would like to discuss >>> how kernel module tracers are expected to implement kvm events probes without >>> the kvm_x86_ops symbol ? >> >> The conversion is done in the TP_fast_assign snippets, which are part of >> kvm.ko and therefore do not need the export. As I understand it, the >> issue is that LTTng cannot use the TP_fast_assign snippets, because they >> are embedded in the trace_event_raw_event_* symbols? > > Indeed, the fact that the TP_fast_assign snippets are embedded in the > trace_event_raw_event_* symbols is an issue for LTTng. This ties those > to ftrace. > > AFAIK, TP_fast_assign copies directly into ftrace ring buffers, and then > afterwards things like dynamic filters are applied, which then "uncommits" the > events if need be (and if possible). Also, TP_fast_assign is tied to the > ftrace ring buffer event layout. The fact that the TP_STRUCT__entry() > (description) > and TP_fast_assign() (open-coded C) are separate fields really focuses on a > use-case where all data is serialized to a ring buffer. > > In LTTng, the event fields are made available to a filter interpreter prior to > being copied into LTTng's ring buffer. This is made possible by implementing > our own LTTNG_TRACEPOINT_EVENT code generation headers. In addition, we have > recently released an event notification mechanism (lttng 2.13) which captures > specific event fields to send with an immediate notification (thus bypassing the > tracer buffering). We are also currently working on a LTTng trace hit counters > mechanism, which performs aggregation through per-cpu counters, which doesn't > even allocate a ring buffer. > > For those reasons, LTTng reimplements its own tracepoint probe callbacks. All > those sit within LTTng kernel modules, which means we currently need the > exported > kvm_x86_ops callbacks. > >> We cannot do the extraction before calling trace_kvm_exit, because it's >> expensive. > > I suspect that extracting relevant data prior to calling trace_kvm_exit > is too expensive because it cannot be skipped when the tracepoint is > disabled. This is because trace_kvm_exit() is a static inline function, > and the check to figure out if the event is enabled is within that function. > Unfortunately, even if the tracepoint is disabled, the side-effects of the > parameters passed to trace_kvm_exit() must happen. > > I've solved this in LTTng-UST by implementing a lttng_ust_tracepoint() > macro, which basically "lifts" the tracepoint enabled check before the > evaluation of the arguments. > > You could achieve something similar by using trace_kvm_exit_enabled() in the > kernel like so: > > if (trace_kvm_exit_enabled()) > trace_kvm_exit(....); > > Which would skip evaluation of the argument side-effects when the tracepoint is > disabled. > > By doing that, when multiple tracers are attached to a kvm tracepoint, the > translation from pointer-to-internal-structure to meaningful fields would only > need to be done once when a tracepoint is hit. And this would remove the need > for using kvm_x86_ops callbacks from tracer probe functions. > > Thoughts ? Hi Paolo, We are at 5.18-rc4 now. Should I expect this unexport to stay in place for 5.18 final and go ahead with using kallsyms to find this symbol from lttng-modules instead ? Thanks, Mathieu > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com > _______________________________________________ > lttng-dev mailing list > lttng-dev@lists.lttng.org > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com