Re: [Xen-devel] Shattering superpages impact on IOMMU in Xen

[Julien Grall <julien.grall@xxxxxxx>]

Hi Oleksandr,

On 04/06/2017 07:59 PM, Oleksandr Tyshchenko wrote:
> Hi, guys.
>
> Seems, it was only my fault. The issue wasn't exactly in shattering,
> the shattering just increased probability for IOMMU page faults to
> occur. I didn't do clean_dcache for the page table entry after
> updating it. So, with clean_dcache I don't see page faults when
> shattering superpages!
> BTW, can I configure domheap pages (which I am using for the IOMMU
> page table) to be uncached? What do you think?
I am not sure if you suggest to configure all the domheap pages to be 
uncached or only a limited number.
In the case where you switch all domheap to uncached, you will have some 
trouble when copy data to/from the guest in hypercall because of 
mismatch attribute.
In the case where you only configure some of the domheap pages, you will 
remove the advantage of 1GB mapping of the domheap in the hypervisor 
table and will increase the memory usage of Xen. Also, you will have to 
be careful when switching back and forth the domheap memory attribute 
between cache and uncache.
If the IOMMU is not able to snoop the cache, then the way forward is to 
use a clean_dcache operation after writing a page table entry. This is 
how we deal in the p2m code.
Cheers,

--
Julien Grall

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