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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 EBB0EC2BA2B for ; Wed, 8 Apr 2020 23:42:16 +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 B710520730 for ; Wed, 8 Apr 2020 23:42:16 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B710520730 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 83FF521548; Wed, 8 Apr 2020 23:42:16 +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 kSxDuklfU+us; Wed, 8 Apr 2020 23:42:15 +0000 (UTC) Received: from lists.linuxfoundation.org (lf-lists.osuosl.org [140.211.9.56]) by silver.osuosl.org (Postfix) with ESMTP id 304C6204C7; Wed, 8 Apr 2020 23:42:15 +0000 (UTC) Received: from lf-lists.osuosl.org (localhost [127.0.0.1]) by lists.linuxfoundation.org (Postfix) with ESMTP id 13B8AC1AE8; Wed, 8 Apr 2020 23:42:15 +0000 (UTC) Received: from fraxinus.osuosl.org (smtp4.osuosl.org [140.211.166.137]) by lists.linuxfoundation.org (Postfix) with ESMTP id 42A23C0177 for ; Wed, 8 Apr 2020 23:42:13 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by fraxinus.osuosl.org (Postfix) with ESMTP id 31D8C85FCB for ; Wed, 8 Apr 2020 23:42:13 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from fraxinus.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id RO5ucoaLRClA for ; Wed, 8 Apr 2020 23:42:12 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by fraxinus.osuosl.org (Postfix) with ESMTPS id 5F04C85F52 for ; Wed, 8 Apr 2020 23:42:12 +0000 (UTC) IronPort-SDR: dc9rlfVFOtT6dbEenIQguH8gktcnrvWK9HrX0qCaSJOQ5/HHU8UP5J1khMlAGRhBzowPl2Ec06 xfN3btLR3aew== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Apr 2020 16:42:11 -0700 IronPort-SDR: tykVs9TctuQLBslfBqGSapkQN5yHP+UzA1hIM6p0f2VDUJOShWOuxwcCu/O6qAwt7V9qqDQNkr SvckWZwRTq+Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,360,1580803200"; d="scan'208";a="398368350" Received: from jacob-builder.jf.intel.com (HELO jacob-builder) ([10.7.199.155]) by orsmga004.jf.intel.com with ESMTP; 08 Apr 2020 16:42:11 -0700 Date: Wed, 8 Apr 2020 16:48:02 -0700 From: Jacob Pan To: Jason Gunthorpe Subject: Re: [PATCH 0/2] iommu: Remove iommu_sva_ops::mm_exit() Message-ID: <20200408164802.155a69e3@jacob-builder> In-Reply-To: <20200408223218.GC11886@ziepe.ca> References: <20200408140427.212807-1-jean-philippe@linaro.org> <20200408113552.7888bfee@jacob-builder> <20200408190226.GA11886@ziepe.ca> <20200408143552.57f5837c@jacob-builder> <20200408223218.GC11886@ziepe.ca> Organization: OTC X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Cc: Jean-Philippe Brucker , arnd@arndb.de, "Yu, Fenghua" , gregkh@linuxfoundation.org, iommu@lists.linux-foundation.org, 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 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. > > 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] _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu