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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8D02BC2BA2B for ; Thu, 9 Apr 2020 14:08:42 +0000 (UTC) Received: from silver.osuosl.org (smtp3.osuosl.org [140.211.166.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 4FB4B20730 for ; Thu, 9 Apr 2020 14:08:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4FB4B20730 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=iommu-bounces@lists.linux-foundation.org Received: from localhost (localhost [127.0.0.1]) by silver.osuosl.org (Postfix) with ESMTP id 1645324B3A; Thu, 9 Apr 2020 14:08:42 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from silver.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id BidKJ32MlXzm; Thu, 9 Apr 2020 14:08:38 +0000 (UTC) Received: from lists.linuxfoundation.org (lf-lists.osuosl.org [140.211.9.56]) by silver.osuosl.org (Postfix) with ESMTP id 9E902203BB; Thu, 9 Apr 2020 14:08:38 +0000 (UTC) Received: from lf-lists.osuosl.org (localhost [127.0.0.1]) by lists.linuxfoundation.org (Postfix) with ESMTP id 91EE6C089F; Thu, 9 Apr 2020 14:08:38 +0000 (UTC) Received: from silver.osuosl.org (smtp3.osuosl.org [140.211.166.136]) by lists.linuxfoundation.org (Postfix) with ESMTP id CFC1AC0177 for ; Thu, 9 Apr 2020 14:08:36 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by silver.osuosl.org (Postfix) with ESMTP id 9E153203BB for ; Thu, 9 Apr 2020 14:08:35 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from silver.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id VRch-3vBPXnY for ; Thu, 9 Apr 2020 14:08:34 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by silver.osuosl.org (Postfix) with ESMTPS id 3047820449 for ; Thu, 9 Apr 2020 14:08:34 +0000 (UTC) IronPort-SDR: lRtK/B+OdxCTR9Kc3eMNB0iNXBxE7xtFRnTPoDX4gH+aUHNqDEa9eh7MzV6FCO5A2WQj7ZEr6R ca38HZ4ou9RA== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Apr 2020 07:08:33 -0700 IronPort-SDR: dt9uFhbpbzQE2SppFY7xvwPd7a9KjYmkjMHS8yJ8SLq4GgGMxrXAwGC68xges7T5xzCOT4qa46 CtGPdg/CbGYQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,362,1580803200"; d="scan'208";a="425516630" Received: from jacob-builder.jf.intel.com (HELO jacob-builder) ([10.7.199.155]) by orsmga005.jf.intel.com with ESMTP; 09 Apr 2020 07:08:33 -0700 Date: Thu, 9 Apr 2020 07:14:24 -0700 From: Jacob Pan To: Jean-Philippe Brucker Subject: Re: [PATCH 0/2] iommu: Remove iommu_sva_ops::mm_exit() Message-ID: <20200409071424.1653b889@jacob-builder> In-Reply-To: <20200409063905.GA2435@myrica> References: <20200408140427.212807-1-jean-philippe@linaro.org> <20200408113552.7888bfee@jacob-builder> <20200408190226.GA11886@ziepe.ca> <20200408143552.57f5837c@jacob-builder> <20200408223218.GC11886@ziepe.ca> <20200408164802.155a69e3@jacob-builder> <20200409063905.GA2435@myrica> Organization: OTC X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Cc: "Yu, Fenghua" , arnd@arndb.de, gregkh@linuxfoundation.org, iommu@lists.linux-foundation.org, Jason Gunthorpe , zhangfei.gao@linaro.org, linux-accelerators@lists.ozlabs.org X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On Thu, 9 Apr 2020 08:39:05 +0200 Jean-Philippe Brucker wrote: > On Wed, Apr 08, 2020 at 04:48:02PM -0700, Jacob Pan wrote: > > On Wed, 8 Apr 2020 19:32:18 -0300 > > Jason Gunthorpe wrote: > > > > > On Wed, Apr 08, 2020 at 02:35:52PM -0700, Jacob Pan wrote: > > > > > On Wed, Apr 08, 2020 at 11:35:52AM -0700, Jacob Pan wrote: > > > > > > Hi Jean, > > > > > > > > > > > > On Wed, 8 Apr 2020 16:04:25 +0200 > > > > > > Jean-Philippe Brucker wrote: > > > > > > > > > > > > > The IOMMU SVA API currently requires device drivers to > > > > > > > implement an mm_exit() callback, which stops device jobs > > > > > > > that do DMA. This function is called in the release() MMU > > > > > > > notifier, when an address space that is shared with a > > > > > > > device exits. > > > > > > > > > > > > > > It has been noted several time during discussions about > > > > > > > SVA that cancelling DMA jobs can be slow and complex, and > > > > > > > doing it in the release() notifier might cause > > > > > > > synchronization issues (patch 2 has more background). > > > > > > > Device drivers must in any case call unbind() to remove > > > > > > > their bond, after stopping DMA from a more favorable > > > > > > > context (release of a file descriptor). > > > > > > > > > > > > > > So after mm exits, rather than notifying device drivers, > > > > > > > we can hold on to the PASID until unbind(), ask IOMMU > > > > > > > drivers to silently abort DMA and Page Requests in the > > > > > > > meantime. This change should relieve the mmput() > > > > > > > path. > > > > > > > > > > > > I assume mm is destroyed after all the FDs are closed > > > > > > > > > > FDs do not hold a mmget(), but they may hold a mmgrab(), ie > > > > > anything using mmu_notifiers has to hold a grab until the > > > > > notifier is destroyed, which is often triggered by FD close. > > > > > > > > > Sorry, I don't get this. Are you saying we have to hold a > > > > mmgrab() between svm_bind/mmu_notifier_register and > > > > svm_unbind/mmu_notifier_unregister? > > > > > > Yes. This is done automatically for the caller inside the > > > mmu_notifier implementation. We now even store the mm_struct > > > pointer inside the notifier. > > > > > > Once a notifier is registered the mm_struct remains valid memory > > > until the notifier is unregistered. > > > > > > > Isn't the idea of mmu_notifier is to avoid holding the mm > > > > reference and rely on the notifier to tell us when mm is going > > > > away? > > > > > > The notifier only holds a mmgrab(), not a mmget() - this allows > > > exit_mmap to proceed, but the mm_struct memory remains. > > > > > > This is also probably why it is a bad idea to tie the lifetime of > > > something like a pasid to the mmdrop as a evil user could cause a > > > large number of mm structs to be released but not freed, probably > > > defeating cgroup limits and so forth (not sure) > > > > > > > It seems both Intel and AMD iommu drivers don't hold mmgrab > > > > after mmu_notifier_register. > > > > > > It is done internally to the implementation. > > > > > > > > So the exit_mmap() -> release() may happen before the FDs are > > > > > destroyed, but the final mmdrop() will be during some FD > > > > > release when the final mmdrop() happens. > > > > > > > > Do you mean mmdrop() is after FD release? > > > > > > Yes, it will be done by the mmu_notifier_unregister(), which > > > should be called during FD release if the iommu lifetime is > > > linked to some FD. > > > > If so, unbind is called in FD release should take care of > > > > everything, i.e. stops DMA, clear PASID context on IOMMU, flush > > > > PRS queue etc. > > > > > > Yes, this is the proper way, when the DMA is stopped and no use > > > of the PASID remains then you can drop the mmu notifier and > > > release the PASID entirely. If that is linked to the lifetime of > > > the FD then forget completely about the mm_struct lifetime, it > > > doesn't matter.. > > Got everything above, thanks a lot. > > > > If everything is in order with the FD close. Why do we need to > > "ask IOMMU drivers to silently abort DMA and Page Requests in the > > meantime." in mm_exit notifier? This will be done orderly in unbind > > anyway. > > When the process is killed, mm release can happen before fds are > released. If you look at do_exit() in kernel/exit.c: > > exit_mm() > mmput() > -> mmu release notifier > ... > exit_files() > close_files() > fput() > exit_task_work() > __fput() > -> unbind() > So unbind is coming anyway, the difference in handling in mmu release notifier is whether we silently drop DMA fault vs. reporting fault? If a process crash during unbind, something already went seriously wrong, DMA fault is expected. I think having some error indication is useful, compared to "silently drop" Thanks, Jacob > Thanks, > Jean > > > > > > > Enforcing unbind upon FD close might be a precarious path, > > > > perhaps that is why we have to deal with out of order > > > > situation? > > > > > > How so? You just put it in the FD release function :) > > > > > I was thinking some driver may choose to defer unbind in some > > workqueue etc. > > > > > > > > In VT-d, because of enqcmd and lazy PASID free we plan to > > > > > > hold on to the PASID until mmdrop. > > > > > > https://lore.kernel.org/patchwork/patch/1217762/ > > > > > > > > > > Why? The bind already gets a mmu_notifier which has refcounts > > > > > and the right lifetime for PASID.. This code could already be > > > > > simplified by using the mmu_notifier_get()/put() stuff. > > > > > > > > > Yes, I guess mmu_notifier_get()/put() is new :) > > > > +Fenghua > > > > > > I was going to convert the intel code when I did many other > > > drivers, but it was a bit too complex.. > > > > > > But the approach is straightforward. Get rid of the mm search > > > list and use mmu_notifier_get(). This returns a singlton notifier > > > for the mm_struct and handles refcounting/etc > > > > > > Use mmu_notifier_put() during a unbind, it will callback to > > > free_notifier() to do the final frees (ie this is where the pasid > > > should go away) > > > > > > For the SVM_FLAG_PRIVATE_PASID continue to use > > > mmu_notifier_register, however this can now be mixed with > > > mmu_notifier_put() so the cleanup is the same. A separate ops > > > static struct is needed to create a unique key though > > > > > > Jason > > > > [Jacob Pan] [Jacob Pan] _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu