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=-15.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable 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 EF467C4338F for ; Tue, 27 Jul 2021 14:00:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D32EE61AAB for ; Tue, 27 Jul 2021 14:00:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236658AbhG0OAo (ORCPT ); Tue, 27 Jul 2021 10:00:44 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:7069 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230500AbhG0OAi (ORCPT ); Tue, 27 Jul 2021 10:00:38 -0400 Received: from dggeme758-chm.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4GYyw04w68zYdMH; Tue, 27 Jul 2021 21:54:40 +0800 (CST) Received: from [10.67.103.235] (10.67.103.235) by dggeme758-chm.china.huawei.com (10.3.19.104) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2176.2; Tue, 27 Jul 2021 22:00:35 +0800 Subject: Re: [PATCH V6 7/8] PCI: Add "pci=disable_10bit_tag=" parameter for peer-to-peer support To: Logan Gunthorpe , Leon Romanovsky References: <1627038402-114183-1-git-send-email-liudongdong3@huawei.com> <1627038402-114183-8-git-send-email-liudongdong3@huawei.com> CC: , , , , , , , From: Dongdong Liu Message-ID: <12c7f276-6869-a432-a138-4fce88da87e3@huawei.com> Date: Tue, 27 Jul 2021 22:00:35 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.67.103.235] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggeme758-chm.china.huawei.com (10.3.19.104) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2021/7/26 23:48, 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. > >>> >>> 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. > >>> >>> 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. 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. So 10-Bit Tag Requester Enable should be set before probe the device driver. Thanks, Dongdong > > Logan > . >