From nobody Mon Feb 9 03:11:14 2026 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 5D09DEB64D9 for ; Wed, 28 Jun 2023 02:27:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230007AbjF1C12 (ORCPT ); Tue, 27 Jun 2023 22:27:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231178AbjF1C1V (ORCPT ); Tue, 27 Jun 2023 22:27:21 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3AA6D30DB for ; Tue, 27 Jun 2023 19:26:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1687919172; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=yRD4FcW0JYERyUR4UO/FaNETlwGZlbYBZCGtLwHwMns=; b=bw4SO1nLuC8DqFllUs6y7K3ZWTIW71OHvKHTFoWn6IisNBK8FGtMhUf6y7EAiq2F85Lg3J /5oetK0Njh1paerHHri8OtPWXRAbfkhrlBud1Rpq8BDxYQue7V10HWqkwzzOxNMSC1J11g cO63xl6K0wi9qCTZ5bgZ4md3+dEB/fw= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-502-U-jPDluIOhGLFJuJDl7bag-1; Tue, 27 Jun 2023 22:26:07 -0400 X-MC-Unique: U-jPDluIOhGLFJuJDl7bag-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B139F3806629; Wed, 28 Jun 2023 02:26:06 +0000 (UTC) Received: from llong.com (unknown [10.22.33.155]) by smtp.corp.redhat.com (Postfix) with ESMTP id D7766207B2C3; Wed, 28 Jun 2023 02:26:05 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Josh Poimboeuf , Pawan Gupta , Jacob Pan , Len Brown , Jonathan Corbet , "Rafael J . Wysocki" Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, x86@kernel.org, linux-pm@vger.kernel.org, Robin Jarry , Joe Mario , Waiman Long Subject: [PATCH v4 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS Date: Tue, 27 Jun 2023 22:25:54 -0400 Message-Id: <20230628022554.1638318-5-longman@redhat.com> In-Reply-To: <20230628022554.1638318-1-longman@redhat.com> References: <20230628022554.1638318-1-longman@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle") disables IBRS when the cstate is 6 or lower. However, there are some use cases where a customer may want to use max_cstate=3D1 to lower latency. Such use cases will suffer from the performance degradation caused by the enabling of IBRS in the sibling idle thread. Add a "ibrs_off" module parameter to force disable IBRS and the CPUIDLE_FLAG_IRQ_ENABLE flag if set. In the case of a Skylake server with max_cstate=3D1, this new ibrs_off option will likely increase the IRQ response latency as IRQ will now be disabled. When running SPECjbb2015 with cstates set to C1 on a Skylake system. First test when the kernel is booted with: "intel_idle.ibrs_off" max-jOPS =3D 117828, critical-jOPS =3D 66047 Then retest when the kernel is booted without the "intel_idle.ibrs_off" added. max-jOPS =3D 116408, critical-jOPS =3D 58958 That means booting with "intel_idle.ibrs_off" improves performance by: max-jOPS: 1.2%, which could be considered noise range. critical-jOPS: 12%, which is definitely a solid improvement. The admin-guide/pm/intel_idle.rst file is updated to add a description about the new "ibrs_off" module parameter. Signed-off-by: Waiman Long --- Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++- drivers/idle/intel_idle.c | 14 ++++++++++++-- 2 files changed, 28 insertions(+), 3 deletions(-) diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/ad= min-guide/pm/intel_idle.rst index b799a43da62e..8604e6d1fe2c 100644 --- a/Documentation/admin-guide/pm/intel_idle.rst +++ b/Documentation/admin-guide/pm/intel_idle.rst @@ -170,7 +170,7 @@ and ``idle=3Dnomwait``. If any of them is present in t= he kernel command line, the ``MWAIT`` instruction is not allowed to be used, so the initialization of ``intel_idle`` will fail. =20 -Apart from that there are four module parameters recognized by ``intel_idl= e`` +Apart from that there are five module parameters recognized by ``intel_idl= e`` itself that can be set via the kernel command line (they cannot be updated= via sysfs, so that is the only way to change their values). =20 @@ -216,6 +216,21 @@ are ignored). The idle states disabled this way can be enabled (on a per-CPU basis) from= user space via ``sysfs``. =20 +The ``ibrs_off`` module parameter is a boolean flag (default to false). It= is +used to control if IBRS (Indirect Branch Restricted Speculation) should be +turned off, if set, when the CPU enters an idle state. This flag will not +affect CPUs that are using Enhanced IBRS which can remain on with little +performance impact. + +For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retb= leed +security vulnerabilities by default. Leaving the IBRS mode on while idlin= g may +have a performance impact on its sibling CPU. The IBRS mode will be turne= d off +by default when the CPU enters into a deep idle state, but not in some +shallower ones. Setting the ``ibrs_off`` module parameter will force the = IBRS +mode to off when the CPU is in any one of the available idle states. This= may +help performance of a sibling CPU at the expense of a slightly higher wake= up +latency for the idle CPU. + =20 .. _intel-idle-core-and-package-idle-states: =20 diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c index e32ff09051af..14874579b396 100644 --- a/drivers/idle/intel_idle.c +++ b/drivers/idle/intel_idle.c @@ -69,6 +69,7 @@ static int max_cstate =3D CPUIDLE_STATE_MAX - 1; static unsigned int disabled_states_mask __read_mostly; static unsigned int preferred_states_mask __read_mostly; static bool force_irq_on __read_mostly; +static bool ibrs_off __read_mostly; =20 static struct cpuidle_device __percpu *intel_idle_cpuidle_devices; =20 @@ -1919,12 +1920,15 @@ static void state_update_enter_method(struct cpuidl= e_state *state, int cstate) } =20 if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) && - state->flags & CPUIDLE_FLAG_IBRS) { + ((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) { /* * IBRS mitigation requires that C-states are entered * with interrupts disabled. */ - WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE); + if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE)) + state->flags &=3D ~CPUIDLE_FLAG_IRQ_ENABLE; + else + WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE); state->enter =3D intel_idle_ibrs; return; } @@ -2346,3 +2350,9 @@ MODULE_PARM_DESC(preferred_cstates, "Mask of preferre= d idle states"); * 'CPUIDLE_FLAG_INIT_XSTATE' and 'CPUIDLE_FLAG_IBRS' flags. */ module_param(force_irq_on, bool, 0444); +/* + * Force the disabling of IBRS when X86_FEATURE_KERNEL_IBRS is on and + * CPUIDLE_FLAG_IRQ_ENABLE isn't set. + */ +module_param(ibrs_off, bool, 0444); +MODULE_PARM_DESC(ibrs_off, "Disable IBRS when idle"); --=20 2.31.1