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=-16.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 EB07BC4338F for ; Tue, 27 Jul 2021 15:41:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D27D561B5F for ; Tue, 27 Jul 2021 15:41:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232549AbhG0Pla (ORCPT ); Tue, 27 Jul 2021 11:41:30 -0400 Received: from mail.kernel.org ([198.145.29.99]:50940 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229506AbhG0Pl3 (ORCPT ); Tue, 27 Jul 2021 11:41:29 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5A40661B53; Tue, 27 Jul 2021 15:41:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627400488; bh=9JFngbViX27YFCZbm+3u4TkuQ+lxMmT7wrLTRSbVfHQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=KA8wvbfUDkMNypag39fC4om9HEtRcuWrRr94Xm3bgb0pqOf8QpdVMwG0KoLKYlp2o 9OmqmxOcB/Sgqe9k6RIXILy/e4rA0X2ztI4NV9Rib+0nTxG1vftMi8+Pu8SeKVxDkf C37JigWIjWBtrJ/1tROUygs6SxgG3RVqeBfYPpVxTF9np8Azt8XWrPJb5tqu5/3+lh PVzdPAvAbK/1RgAK3ocOLH+JZ420Ox/970F0WnQ6jvTvNlCp99SvvmjHC6nuQZB7fg lthn1QKJyXXcInhxiXo5AyIqUWx0T+CSMvYcYIh6NoZvI+sK+NoZkdWHz75T6MxUEy PenXsAE72LsiQ== Date: Tue, 27 Jul 2021 18:41:24 +0300 From: Leon Romanovsky To: Dongdong Liu Cc: Logan Gunthorpe , helgaas@kernel.org, hch@infradead.org, kw@linux.com, linux-pci@vger.kernel.org, rajur@chelsio.com, hverkuil-cisco@xs4all.nl, linux-media@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH V6 7/8] PCI: Add "pci=disable_10bit_tag=" parameter for peer-to-peer support Message-ID: References: <1627038402-114183-1-git-send-email-liudongdong3@huawei.com> <1627038402-114183-8-git-send-email-liudongdong3@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Tue, Jul 27, 2021 at 10:30:40PM +0800, Dongdong Liu wrote: > > > On 2021/7/27 19:05, Leon Romanovsky wrote: > > On Mon, Jul 26, 2021 at 09:48:57AM -0600, Logan Gunthorpe wrote: > > > > > > > > > On 2021-07-25 12:39 a.m., Leon Romanovsky wrote: > > > > On Fri, Jul 23, 2021 at 10:20:50AM -0600, Logan Gunthorpe wrote: > > > > > > > > > > > > > > > > > > > > On 2021-07-23 5:32 a.m., Leon Romanovsky wrote: > > > > > > On Fri, Jul 23, 2021 at 07:06:41PM +0800, Dongdong Liu wrote: > > > > > > > PCIe spec 5.0 r1.0 section 2.2.6.2 says that if an Endpoint supports > > > > > > > sending Requests to other Endpoints (as opposed to host memory), the > > > > > > > Endpoint must not send 10-Bit Tag Requests to another given Endpoint > > > > > > > unless an implementation-specific mechanism determines that the Endpoint > > > > > > > supports 10-Bit Tag Completer capability. Add "pci=disable_10bit_tag=" > > > > > > > parameter to disable 10-Bit Tag Requester if the peer device does not > > > > > > > support the 10-Bit Tag Completer. This will make P2P traffic safe. > > > > > > > > > > > > > > Signed-off-by: Dongdong Liu > > > > > > > --- > > > > > > > Documentation/admin-guide/kernel-parameters.txt | 7 ++++ > > > > > > > drivers/pci/pci.c | 56 +++++++++++++++++++++++++ > > > > > > > drivers/pci/pci.h | 1 + > > > > > > > drivers/pci/pcie/portdrv_pci.c | 13 +++--- > > > > > > > drivers/pci/probe.c | 9 ++-- > > > > > > > 5 files changed, 78 insertions(+), 8 deletions(-) > > > > > > > > > > > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > > > > > > index bdb2200..c2c4585 100644 > > > > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > > > > @@ -4019,6 +4019,13 @@ > > > > > > > bridges without forcing it upstream. Note: > > > > > > > this removes isolation between devices and > > > > > > > may put more devices in an IOMMU group. > > > > > > > + disable_10bit_tag=[; ...] > > > > > > > + Specify one or more PCI devices (in the format > > > > > > > + specified above) separated by semicolons. > > > > > > > + Disable 10-Bit Tag Requester if the peer > > > > > > > + device does not support the 10-Bit Tag > > > > > > > + Completer.This will make P2P traffic safe. > > > > > > > > > > > > I can't imagine more awkward user experience than such kernel parameter. > > > > > > > > > > > > As a user, I will need to boot the system, hope for the best that system > > > > > > works, write down all PCI device numbers, guess which one doesn't work > > > > > > properly, update grub with new command line argument and reboot the > > > > > > system. Any HW change and this dance should be repeated. > > > > > > > > > > There are already two such PCI parameters with this pattern and they are > > > > > not that awkward. pci_dev may be specified with either vendor/device IDS > > > > > or with a path of BDFs (which protects against renumbering). > > > > > > > > Unfortunately, in the real world, BDF is not so stable. It changes with > > > > addition of new hardware, BIOS upgrades and even broken servers. > > > > > > That's why it supports using a *path* of BDFs which tends not to catch > > > the wrong device if the topology changes. > > > > > > > Vendor/device IDs doesn't work if you have multiple devices of same > > > > vendor in the system. > > > > > > Yes, but it's fine for some use cases. That's why there's a range of > > > options. > > > > The thing is that you are adding PCI parameter that is applicable to everyone. > > > > We probably see different usage models for this feature. In my world, users > > have thousands of servers that runs 24x7, with VMs on top, some of them perform > > FW upgrades without stopping anything. The idea that you can reboot such server > > any time, simply doesn't exist. > > > > So if I need to enable/disable this feature for one of the VFs, I will be stuck. > > > > > > > > > > > > > > > This flag is only useful in P2PDMA traffic, and if the user attempts > > > > > such a transfer, it prints a warning (see the next patch) with the exact > > > > > parameter that needs to be added to the command line. > > > > > > > > Dongdong citied PCI spec and it was very clear - don't enable this > > > > feature unless you clearly know that it is safe to enable. This is > > > > completely opposite to the proposal here - always enable and disable > > > > if something is printed to the dmesg. > > > > > > Quoting from patch 4: > > > > > > "For platforms where the RC supports 10-Bit Tag Completer capability, > > > it is highly recommended for platform firmware or operating software > > > that configures PCIe hierarchies to Set the 10-Bit Tag Requester Enable > > > bit automatically in Endpoints with 10-Bit Tag Requester capability. > > > This enables the important class of 10-Bit Tag capable adapters that > > > send Memory Read Requests only to host memory." > > > > > > Notice the last sentence. It's saying that devices who only talk to host > > > memory should have 10-bit tags enabled. In the kernel we call devices > > > that talk to things besides host memory "P2PDMA". So the spec is saying > > > not to enable 10bit tags for devices participating in P2PDMA. The kernel > > > needs a way to allow users to do that. The kernel parameter only stops > > > the feature from being enabled for a specific device, and the only > > > use-case is P2PDMA which is not that common and requires the user to be > > > aware of their topology. So I really don't think this is that big a problem. > > > > I'm not question the feature and the need of configuration. My concern > > is just *how* this feature is configured. > > > > > > > > > > > > > > > This has worked well for disable_acs_redir and was used for > > > > > resource_alignment before that for quite some time. So save a better > > > > > suggestion I think this is more than acceptable. > > > > > > > > I don't know about other parameters and their history, but we are not in > > > > 90s anymore and addition of modules parameters (for the PCI it is kernel > > > > cmdline arguments) are better to be changed to some configuration tool/sysfs. > > > > > > The problem was that the ACS bits had to be set before the kernel > > > enumerated the devices. The IOMMU code simply was not able to support > > > dynamic adjustments to its groups. I assume changing 10bit tags > > > dynamically is similarly tricky -- but if it's not then, yes a sysfs > > > interface in addition to the kernel parameter would be a good idea. > > > > I think that it is doable with combination of drivers_autoprobe disable > > and some sysfs knob to enable/disable this feature before driver bind. > > > > It should be very similar to that we did for the dynamic MSI-X, see > > /sys/bus/pci/devices/.../sriov_vf_msix_count > > Many thanks for your suggestion. > > Seems a sysfs could be work ok, but need to make sure 10-Bit Tag Requester > to be set before binding the device driver as > PCIe spec 5.0 section 7.5.3.16 Device Control 2 Register > 10-Bit Tag Requester Enable says that > If software changes the value of this bit while the Function > has outstanding Non-Posted Requests, the result is undefined. This is where drivers_autoprobe will help. Thanks > > Thanks, > Dongdong > > > > Thanks > > > > > > > > Logan > > . > >