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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 C517AC4360F for ; Tue, 2 Apr 2019 14:40:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9DF3120882 for ; Tue, 2 Apr 2019 14:40:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732151AbfDBOks (ORCPT ); Tue, 2 Apr 2019 10:40:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51018 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730673AbfDBOks (ORCPT ); Tue, 2 Apr 2019 10:40:48 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 34937C04FFEF; Tue, 2 Apr 2019 14:40:48 +0000 (UTC) Received: from x1.home (ovpn-116-99.phx2.redhat.com [10.3.116.99]) by smtp.corp.redhat.com (Postfix) with ESMTP id 7563F8491A; Tue, 2 Apr 2019 14:40:41 +0000 (UTC) Date: Tue, 2 Apr 2019 08:40:40 -0600 From: Alex Williamson To: Peter Xu Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, eric.auger@redhat.com, cohuck@redhat.com Subject: Re: [PATCH] vfio/type1: Limit DMA mappings per container Message-ID: <20190402084040.72c3ceb5@x1.home> In-Reply-To: <20190402051802.GB11008@xz-x1> References: <155414977872.12780.13728555131525362206.stgit@gimli.home> <20190402024115.GA11008@xz-x1> <20190401223413.3783af5f@x1.home> <20190402051802.GB11008@xz-x1> Organization: Red Hat MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Tue, 02 Apr 2019 14:40:48 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2 Apr 2019 13:18:02 +0800 Peter Xu wrote: > On Mon, Apr 01, 2019 at 10:34:13PM -0600, Alex Williamson wrote: > > On Tue, 2 Apr 2019 10:41:15 +0800 > > Peter Xu wrote: > > > > > On Mon, Apr 01, 2019 at 02:16:52PM -0600, Alex Williamson wrote: > > > > > > [...] > > > > > > > @@ -1081,8 +1088,14 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu, > > > > goto out_unlock; > > > > } > > > > > > > > + if (!atomic_add_unless(&iommu->dma_avail, -1, 0)) { > > > > + ret = -ENOSPC; > > > > + goto out_unlock; > > > > + } > > > > + > > > > dma = kzalloc(sizeof(*dma), GFP_KERNEL); > > > > if (!dma) { > > > > + atomic_inc(&iommu->dma_avail); > > > > > > This should be the only special path to revert the change. Not sure > > > whether this can be avoided by simply using atomic_read() or even > > > READ_ONCE() (I feel like we don't need atomic ops with dma_avail > > > because we've had the mutex but it of course it doesn't hurt...) to > > > replace atomic_add_unless() above to check against zero then we do > > > +1/-1 in vfio_[un]link_dma() only. But AFAICT this patch is correct. > > > > Thanks for the review, you're right, we're only twiddling this atomic > > while holding the iommu->lock mutex, so it appears unnecessary. Since > > we're within the mutex, I think we don't even need a READ_ONCE. We can > > simple test it before alloc and decrement after. Am I missing something > > that would specifically require READ_ONCE within our mutex critical > > section? Thanks, > > I don't know very clear on this and I'd be glad to learn about that. > My understanding is that [READ|WRITE]_ONCE() is the same as volatile > mem operation and will make sure we don't keep variables in the > registers. So if the mutex semantics can support that (say, a "*addr > = val" following with a mutex_unlock will make sure "val" will > definitely land into memory of "&addr") then I do think it's fine even > without it (which corresponds to WRITE_ONCE(&addr, val) in this case). The READ/WRITE_ONCE macros add memory barriers, but we have the mutex for protecting concurrent access to the data. I don't see that there's anything special about a counter on the iommu object that needs special attention vs any other elements that might get modified in these sections. Thanks, Alex