Re: [Xen-devel] [PATCH] amd iommu: Do not adjust paging mode for dom0 devices

[Wei Wang2 <wei.wang2@xxxxxxx>]

On Sunday 06 February 2011 17:58:18 Keir Fraser wrote:
> On 01/02/2011 17:34, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:
> > Keir
> > Here is another small patch to fix consistency issue of paging mode when
> > return passthru devices back to dom0. In this case, paging mode should
> > not be adjusted, since dom0->max_pages = ~0U.
>
> The call to get_paging_mode() in amd_iommu_domain_init() looks sensible
> enough. Why is a call needed in reassign_device() at all? Checking for dom0
> in reassign_device just looks like a fragile hack.
>
>  -- Keir

Keir,
amd_iommu_domain_init() is called very early, where get_paging_mode() cannot  
use domU->max_pages to setup proper io page level for domU. Instead, paging 
mode of domU has to be initialized as 4 for safety. That is why I want to 
adjust hd->paging_mode in reassign_device() using domU->max_pages. Since most 
domU use 2-3 level page tables, always using 4 level might be a waste.

But I had a wrong assumption of dom0->max_pages, and consequently when return 
passthru device back to dom0, following lines in c/s 22825

+    if ( target->max_pages > 0 )
+        t->paging_mode = get_paging_mode(target->max_pages);

will change hd->paging_mode of dom0 from 3 (in most case) to 4 (using 
get_paging_mode(dom0->max_pages)). 

Thanks,
-Wei







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