From mboxrd@z Thu Jan 1 00:00:00 1970 From: Baoquan He Subject: Re: [PATCH v8 10/13] iommu/amd: Allocate memory below 4G for dev table if translation pre-enabled Date: Fri, 28 Jul 2017 17:06:19 +0800 Message-ID: <20170728090619.GM24304@x1> References: <1500627551-12930-1-git-send-email-bhe@redhat.com> <1500627551-12930-11-git-send-email-bhe@redhat.com> <20170727155550.GO3610@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170727155550.GO3610@suse.de> Sender: linux-kernel-owner@vger.kernel.org To: Joerg Roedel Cc: iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org List-Id: iommu@lists.linux-foundation.org Hi Joerg, On 07/27/17 at 05:55pm, Joerg Roedel wrote: > On Fri, Jul 21, 2017 at 04:59:08PM +0800, Baoquan He wrote: > > AMD pointed out it's unsafe to update the device-table while iommu > > is enabled. It turns out that device-table pointer update is split > > up into two 32bit writes in the IOMMU hardware. So updating it while > > the IOMMU is enabled could have some nasty side effects. > > > > The only way to work around this is to allocate the device-table below > > 4GB if translation is pre-enabled in kdump kernel. If allocation failed, > > still use the old one. > > Not only for the kdump kernel. The old device table must also be below > 4GB so that its pointer can be updated with a 32bit write. > > If the old table is above 4GB you still need the second write to zero > the upper parts of the pointer in hardware. Do you mean the allocation of amd_iommu_dev_table in early_amd_iommu_init() also need be addressed for 1st kernel? Seems we don't make sure that for 1st kernel, like adding GFP_DMA32 flag when allocate amd_iommu_dev_table in amd_iommu_dev_table early_amd_iommu_init().