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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 904C1C43603 for ; Tue, 17 Dec 2019 19:19:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6CC95218AC for ; Tue, 17 Dec 2019 19:19:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727529AbfLQTT3 (ORCPT ); Tue, 17 Dec 2019 14:19:29 -0500 Received: from mga01.intel.com ([192.55.52.88]:14376 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727328AbfLQTT3 (ORCPT ); Tue, 17 Dec 2019 14:19:29 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 17 Dec 2019 11:19:28 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.69,326,1571727600"; d="scan'208";a="212478542" Received: from chenyian-desk1.amr.corp.intel.com (HELO [10.3.52.63]) ([10.3.52.63]) by fmsmga007.fm.intel.com with ESMTP; 17 Dec 2019 11:19:28 -0800 Subject: Re: [PATCH 1/3] iommu/vt-d: skip RMRR entries that fail the sanity check To: Barret Rhoden , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , David Woodhouse , Joerg Roedel , Sohil Mehta Cc: linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, x86@kernel.org References: <20191211194606.87940-1-brho@google.com> <20191211194606.87940-2-brho@google.com> <99a294a0-444e-81f9-19a2-216aef03f356@intel.com> <93820c21-8a37-d8f0-dacb-29cee694a91d@google.com> From: "Chen, Yian" Message-ID: <4c24f2d2-03fd-a6cb-f950-391f3f7837cb@intel.com> Date: Tue, 17 Dec 2019 11:19:28 -0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <93820c21-8a37-d8f0-dacb-29cee694a91d@google.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/16/2019 11:35 AM, Barret Rhoden wrote: > On 12/16/19 2:07 PM, Chen, Yian wrote: >> >> >> On 12/11/2019 11:46 AM, Barret Rhoden wrote: >>> RMRR entries describe memory regions that are DMA targets for devices >>> outside the kernel's control. >>> >>> RMRR entries that fail the sanity check are pointing to regions of >>> memory that the firmware did not tell the kernel are reserved or >>> otherwise should not be used. >>> >>> Instead of aborting DMAR processing, this commit skips these RMRR >>> entries.  They will not be mapped into the IOMMU, but the IOMMU can >>> still be utilized.  If anything, when the IOMMU is on, those devices >>> will not be able to clobber RAM that the kernel has allocated from >>> those >>> regions. >>> >>> Signed-off-by: Barret Rhoden >>> --- >>>   drivers/iommu/intel-iommu.c | 2 +- >>>   1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c >>> index f168cd8ee570..f7e09244c9e4 100644 >>> --- a/drivers/iommu/intel-iommu.c >>> +++ b/drivers/iommu/intel-iommu.c >>> @@ -4316,7 +4316,7 @@ int __init dmar_parse_one_rmrr(struct >>> acpi_dmar_header *header, void *arg) >>>       rmrr = (struct acpi_dmar_reserved_memory *)header; >>>       ret = arch_rmrr_sanity_check(rmrr); >>>       if (ret) >>> -        return ret; >>> +        return 0; >>>       rmrru = kzalloc(sizeof(*rmrru), GFP_KERNEL); >>>       if (!rmrru) >> Parsing rmrr function should report the error to caller. The behavior >> to response the error can be >> chose  by the caller in the calling stack, for example, >> dmar_walk_remapping_entries(). >> A concern is that ignoring a detected firmware bug might have a >> potential side impact though >> it seemed safe for your case. > > That's a little difficult given the current code.  Once we are in > dmar_walk_remapping_entries(), the specific function (parse_one_rmrr) > is called via callback: > >     ret = cb->cb[iter->type](iter, cb->arg[iter->type]); >     if (ret) >         return ret; > > If there's an error of any sort, it aborts the walk.  Handling the > specific errors here is difficult, since we don't know what the errors > mean to the specific callback.  Is there some errno we can use that > means "there was a problem, but it's not so bad that you have to > abort, but I figured you ought to know"?  Not that I think that's a > good idea. > > The knowledge of whether or not a specific error is worth aborting all > DMAR functionality is best known inside the specific callback.  The > only handling to do is print a warning and either skip it or abort. > > I think skipping the entry for a bad RMRR is better than aborting > completely, though I understand if people don't like that.  It's > debatable.  By aborting, we lose the ability to use the IOMMU at all, > but we are still in a situation where the devices using the RMRR > regions might be clobbering kernel memory, right?  Using the IOMMU > (with no mappings for the bad RMRRs) would stop those devices from > clobbering memory. > > Regardless, I have two other patches in this series that could resolve > the problem for me and probably other people.  I'd just like at least > one of the three patches to get merged so that my machine boots when > the original commit f036c7fa0ab6 ("iommu/vt-d: Check VT-d RMRR region > in BIOS is reported as reserved") gets released. > when a firmware bug appears, the potential problem may beyond the scope of its visible impacts so that introducing a workaround in official implementation should be considered very carefully. If the workaround is really needed at this point, I would recommend adding a WARN_TAINT with TAINT_FIRMWARE_WORKAROUND, to tell the workaround is in the place. Thanks Yian > Thanks, > > Barret >