Re: [Xen-devel] [RFC PATCH 9/9] xen: Add use_iommu flag to createdomain domctl

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Apr 19, 2017 at 07:26:44PM +0100, Julien Grall wrote:
> Hi Oleksandr,
> 
> Please CC the appropriate maintainers for all the components you modify.
> 
> On 15/03/17 20:05, Oleksandr Tyshchenko wrote:
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > 
> > This flag is intended to let Xen know that the guest has devices
> > which will most likely be used for passthrough.
> > The primary aim of this knowledge is to help the IOMMUs that don't
> > share page tables with the CPU be ready before P2M code starts
> > updating IOMMU mapping.
> > So, if this flag is set the unshared IOMMUs will populate their
> > page tables at the domain creation time and thereby will be able
> > to handle IOMMU mapping updates from *the very beginning*.
> > 

If it is not able to do so since the very beginning, what will happen?

Let me explain where I'm coming from:

1. if not populating the iommu page table causes Xen to malfunction
   (crash?), it is a bug. It should be fixed without involvement
   from toolstack.
2. if this is an optimasation, can't we not always populate iommu pt
   hence no new flag is needed?

Overall I'm not too convinced a new flag is needed.

Wei.

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