Date: Thu, 27 Jul 2017 17:55:50 +0200
From: Joerg Roedel <jroedel@suse.de>
To: Baoquan He <bhe@redhat.com>
Cc: iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 10/13] iommu/amd: Allocate memory below 4G for dev
 table if translation pre-enabled
Message-ID: <20170727155550.GO3610@suse.de>
References: <1500627551-12930-1-git-send-email-bhe@redhat.com>
 <1500627551-12930-11-git-send-email-bhe@redhat.com>
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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.