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

[Joerg Roedel]

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.