From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 47120C433EF
	for <linux-kernel@archiver.kernel.org>; Fri, 24 Sep 2021 23:12:01 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [23.128.96.18])
	by mail.kernel.org (Postfix) with ESMTP id 2A41961261
	for <linux-kernel@archiver.kernel.org>; Fri, 24 Sep 2021 23:12:01 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S240648AbhIXXNd (ORCPT <rfc822;linux-kernel@archiver.kernel.org>);
        Fri, 24 Sep 2021 19:13:33 -0400
Received: from mga01.intel.com ([192.55.52.88]:51291 "EHLO mga01.intel.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S230021AbhIXXNc (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 24 Sep 2021 19:13:32 -0400
X-IronPort-AV: E=McAfee;i="6200,9189,10117"; a="246659417"
X-IronPort-AV: E=Sophos;i="5.85,321,1624345200"; 
   d="scan'208";a="246659417"
Received: from fmsmga002.fm.intel.com ([10.253.24.26])
  by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Sep 2021 16:11:58 -0700
X-IronPort-AV: E=Sophos;i="5.85,321,1624345200"; 
   d="scan'208";a="559859753"
Received: from agluck-desk2.sc.intel.com (HELO agluck-desk2.amr.corp.intel.com) ([10.3.52.146])
  by fmsmga002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Sep 2021 16:11:58 -0700
Date:   Fri, 24 Sep 2021 16:11:57 -0700
From:   "Luck, Tony" <tony.luck@intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Jacob Jun Pan <jacob.jun.pan@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        iommu@lists.linux-foundation.org,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting
Message-ID: <YU5bPbyBjUrD9S8b@agluck-desk2.amr.corp.intel.com>
References: <20210920192349.2602141-1-fenghua.yu@intel.com>
 <20210920192349.2602141-6-fenghua.yu@intel.com>
 <87y27nfjel.ffs@tglx>
 <YUyuEjlrcOeCp4qQ@agluck-desk2.amr.corp.intel.com>
 <87o88jfajo.ffs@tglx>
 <87k0j6dsdn.ffs@tglx>
 <YU3414QT0J7EN4w9@agluck-desk2.amr.corp.intel.com>
 <a77ee33c-6fa7-468c-8fc0-a0a2ce725e75@www.fastmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <a77ee33c-6fa7-468c-8fc0-a0a2ce725e75@www.fastmail.com>
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Sep 24, 2021 at 04:03:53PM -0700, Andy Lutomirski wrote:
> 1. The context switch code needs to resync PASID.  Unfortunately, this adds some overhead to every context switch, although a static_branch could minimize it for non-PASID users.

Any solution that adds to context switch time isn't going to meet
the definition of "perfect" either.

-Tony