From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joerg Roedel Subject: Re: [PATCH 0/8] iommu/vt-d: Fix crash dump failure caused by legacy DMA/IO Date: Wed, 2 Jul 2014 15:32:59 +0200 Message-ID: <20140702133258.GN26537@8bytes.org> References: <1398386198-19304-1-git-send-email-bill.sumner@hp.com> <1398854973.12733.23.camel@i7.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1398854973.12733.23.camel-W2I5cNIroUsVm/YvaOjsyQ@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: David Woodhouse Cc: bhe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, linux-pci-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kexec-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, doug.hatch-VXdhtT5mjnY@public.gmane.org, ishii.hironobu-+CUm20s59erQFUHtdCDX3A@public.gmane.org, bhelgaas-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, zhenhua-VXdhtT5mjnY@public.gmane.org, Bill Sumner List-Id: iommu@lists.linux-foundation.org Hi David, On Wed, Apr 30, 2014 at 11:49:33AM +0100, David Woodhouse wrote: > There could be all kinds of existing mappings in the DMA page tables, > and I'm not sure it's safe to preserve them. What prevents the crashdump > kernel from trying to use any of the physical pages which are > accessible, and which could thus be corrupted by stray DMA? > > In fact, the old kernel could even have set up 1:1 passthrough mappings > for some devices, which would then be able to DMA *anywhere*. Surely we > need to prevent that? Ideally we would prevent that, yes. But the problem is that a failed DMA transaction might put the device into an unrecoverable state. Usually any in-flight DMA transactions should only target buffers set up by the previous kernel and not corrupt any data. > After the last round of this patchset, we discussed a potential > improvement where you point every virtual bus address at the *same* > physical scratch page. That is a solution to prevent the in-flight DMA failures. But what happens when there is some in-flight DMA to a disk to write some inodes or a new superblock. Then this scratch address-space may cause filesystem corruption at worst. So with this in mind I would prefer initially taking over the page-tables from the old kernel before the device drivers re-initialize the devices. Joerg