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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5ADA6C433EF for ; Fri, 24 Sep 2021 16:12:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 351CD6124D for ; Fri, 24 Sep 2021 16:12:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347308AbhIXQOY (ORCPT ); Fri, 24 Sep 2021 12:14:24 -0400 Received: from mga11.intel.com ([192.55.52.93]:43617 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1347225AbhIXQOX (ORCPT ); Fri, 24 Sep 2021 12:14:23 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10116"; a="220906272" X-IronPort-AV: E=Sophos;i="5.85,320,1624345200"; d="scan'208";a="220906272" Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Sep 2021 09:12:49 -0700 X-IronPort-AV: E=Sophos;i="5.85,320,1624345200"; d="scan'208";a="559676132" Received: from otcwcpicx3.sc.intel.com ([172.25.55.73]) by fmsmga002-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Sep 2021 09:12:49 -0700 Date: Fri, 24 Sep 2021 16:12:43 +0000 From: Fenghua Yu To: Thomas Gleixner Cc: "Luck, Tony" , Ingo Molnar , Borislav Petkov , Peter Zijlstra , Andy Lutomirski , Dave Hansen , Lu Baolu , Joerg Roedel , Josh Poimboeuf , Dave Jiang , Jacob Jun Pan , Ashok Raj , Ravi V Shankar , iommu@lists.linux-foundation.org, x86 , linux-kernel Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting Message-ID: References: <20210920192349.2602141-1-fenghua.yu@intel.com> <20210920192349.2602141-6-fenghua.yu@intel.com> <87y27nfjel.ffs@tglx> <87o88jfajo.ffs@tglx> <87k0j6dsdn.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k0j6dsdn.ffs@tglx> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Thomas, On Fri, Sep 24, 2021 at 03:18:12PM +0200, Thomas Gleixner wrote: > On Thu, Sep 23 2021 at 19:48, Thomas Gleixner wrote: > > On Thu, Sep 23 2021 at 09:40, Tony Luck wrote: > > > > fpu_write_task_pasid() can just grab the pasid from current->mm->pasid > > and be done with it. > > > > The task exit code can just call iommu_pasid_put_task_ref() from the > > generic code and not from within x86. > > But OTOH why do you need a per task reference count on the PASID at all? > > The PASID is fundamentaly tied to the mm and the mm can't go away before > the threads have gone away unless this magically changed after I checked > that ~20 years ago. There are up to 1M PASIDs because PASID is 20-bit. I think there are a few ways to allocate and free PASID: 1. Statically allocate a PASID once a mm is created and free it in mm exit. No PASID allocation/free during the mm's lifetime. Then up to 1M processes can be created due to 1M PASIDs limitation. We don't want this method because the 1M processes limitation. 2. A PASID is allocated to the mm in open(dev)->bind(dev, mm). There are three ways to free it: (a) Actively free it in close(fd)->unbind(dev, mm) by sending IPIs to tell all tasks using the PASID to clear the IA32_PASID MSR. This has locking issues similar to the actively loading IA32_PASID MSR which was force disabled in upstream. So won't work. (b) Passively free the PASID in destroy_context(mm) in mm exit. Once the PASID is allocated, it stays with the process for the lifetime. It's better than #1 because the PASID is allocated only on demand. (c) Passively free the PASID in deactive_mm(mm) or unbind() whenever there is no usage as implemented in this series. Tracking the PASID usage per task provides a chance to free the PASID on task exit. The PASID has a better chance to be freed earlier than mm exit in #(b). This series uses #2 and #(c) to allocate and free the PASID for a better chance to ease the 1M PASIDs limitation pressure. For example, a thread doing open(dev)->ENQCMD->close(fd)->exit(2) will not occupy a PASID while its sibling threads are still running. Thanks. -Fenghua