Re: [Xen-devel] question on iommu _map_page

[Kaushik Kumar Ram <kaushik@xxxxxxxx>]

On May 28, 2010, at 3:08 AM, Keir Fraser wrote:

> On 27/05/2010 21:59, "Kaushik Kumar Ram" <kaushik@xxxxxxxx> wrote:
> 
>> On May 27, 2010, at 3:46 PM, Keir Fraser wrote:
>> 
>>> It looks like the person who implemented that never needed the read-only
>>> case. We need a function to map pages into the iommu read-only and to call
>>> that from the grant code.
>>> 
>>> A patch to just call iommu_map_page() for any kind of grant mapping would be
>>> acceptable for now, if you want to submit a patch.
>> 
>> OK. Attached.
> 
> Actually I've implemented the better fix as xen-unstable:21476. Hope that
> works okay for you.


Checking if act->pin is zero before calling iommu_unmap_page() (in 
grant_table.c) is not sufficient since there can be multiple
active grants all referring to the same mfn. In fact I came across iommu page 
faults because pages were 
getting unmapped from the IOMMU when active grants referring to these pages 
were still around.
Ideally, there needs to be a per-page count of how many IOMMU mappings exists 
for a page. 
I can't think of an obvious fix for this problem. For my purposes, I hacked 
page_info to add another counter which I guess is not an acceptable solution! 

-Kaushik


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