Re: [Qemu-devel] [PATCH 10/12] spapr_pci: (Mostly) remove spapr-pci-vfio-host-bridge

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Feb 29, 2016 at 12:42:28PM +1100, Alexey Kardashevskiy wrote:
> On 02/26/2016 10:32 PM, David Gibson wrote:
> >Now that the regular spapr-pci-host-bridge can handle EEH, there are only
> >two things that spapr-pci-vfio-host-bridge does differently:
> >     1. automatically sizes its DMA window to match the host IOMMU
> >     2. checks if the attached VFIO container is backed by the
> >        VFIO_SPAPR_TCE_IOMMU type on the host
> >
> >(1) is not particularly useful, since the default window used by the
> >regular host bridge will work with the host IOMMU configuration on all
> >current systems anyway.
> >
> >Plus, automatically changing guest visible configuration (such as the DMA
> >window) based on host settings is generally a bad idea.
> 
> btw why exactly is it a bad idea?

So, as noted, in this case it's probably ok.  But in general changing
configuration based on host state will break migration, because the
source and destination VMs, with apparently identical setup won't
actually be the same.

It also makes it more difficult for management layers to know exactly
what they're constructing, which is potentially an issue even for this
non-migratable device.
> 
> 
> Anyway,
> Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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