linux-kernel - Re: [PATCH v8 10/13] iommu/amd: Allocate memory below 4G for dev table if translation pre-enabled

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Date:   Thu, 27 Jul 2017 17:55:50 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Baoquan He <bhe@...hat.com>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 10/13] iommu/amd: Allocate memory below 4G for dev
 table if translation pre-enabled

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.