Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

[Michael Abd-El-Malek <mabdelmalek@xxxxxxx>]

On Apr 29, 2008, at 1:32 PM, Jeremy Fitzhardinge wrote:
> Michael Abd-El-Malek wrote:
> > Some drivers may need to do additional tasks besides just clearing  
> > the PTE.   For example, when unzapping my kernel PTE, I need to  
> > restore the physical mapping of the page.  (On the initial set_pte  
> > (which I've overridden), I removed the physical backing of the page.)
> > If we really want to avoid a zap_pte hook, I suppose we can add  
> > flags to the page/PTE that indicate things like "this page needs to  
> > have its physical backing restored".
> Is that something that needs to happen synchronously with the unmap,  
> or could the driver do it in its vma callback?  What do you mean by  
> remove/restoring the physical backing?  Do any of the in-tree  
> drivers do this?
Good point.  This extra work can happen later in the vma's callback.

By removing/restoring the physical backing, I mean doing the work in  
balloon.c.  Removing the physical backing is done by doing a  
XENMEM_decrease_reservation hypercall, while restoring the physical  
backing is done through XENMEM_populate_physmap.
> In the PAE case, which I think we can regard as usual these days, we  
> have 64-bits to store per-mapping info, so stashing a callback or  
> callback index seems pretty straightforward if we need full  
> generality.
I don't think there's a need for this, either in the current gntdev/ 
blktap and my work.  The generic xen code can undo the user-space  
grant, and the vma callback can later do any extra work.
Cheers,
Mike

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