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List:       xen-devel
Subject:    Re: [Xen-devel] Does xc_map_foreign_pages() update IOMMU tables?
From:       Joanna Rutkowska <joanna () invisiblethingslab ! com>
Date:       2010-05-13 12:39:53
Message-ID: 4BEBF319.3020202 () invisiblethingslab ! com
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On 05/13/2010 02:27 PM, Keir Fraser wrote:
> On 13/05/2010 13:09, "Joanna Rutkowska" <joanna@invisiblethingslab.com>
> wrote:
> 
>>> Seeing as how xc_map_foreign_pages() maps pages into a particular
>>> guest-process virtual address space, whereas IOMMU tables are a mapping from
>>> guest-physical address space, it's clear that x_m_f_p() cannot sensibly be
>>> having any effect on the IOMMU.
>>>
>> Interesting. That suggests the graphics drivers we tested do not use DMA
>> for XShmPutImage()...
> 
> Ah, actually dom0 is special in that it gets a 1:1 mapping of all of RAM
> (except for some private Xen areas). See
> drivers/passthrough/vtd/x86/vtd.c:iommu_set_dom0_mapping(). I forgot about
> that when I wrote my previous reply!
> 

Ah, indeed! In fact Rafal and I even had this very fragment of code in
our slides 2 years ago and we both forgot about it :) [Back then it was
in drivers/passthrough/vtd/iommu.c:intel_iommu_domain_init().

Thanks for clarifying!

joanna.


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