Re: [Xen-users] Does XEN on ARM support allocation for physical continuous memory on DOM0/U?

[Tom Ting[äéå] <tom_ting@xxxxxxxxxxx>]

>-----Original Message-----
>From: Julien Grall [mailto:julien.grall@xxxxxxxxxx] 
>Sent: Tuesday, October 20, 2015 9:21 PM
>To: Tom Ting[äéå]; Ian Campbell; xen-users@xxxxxxxxxxxxx
>Subject: Re: [Xen-users] Does XEN on ARM support allocation for physical 
>continuous memory on DOM0/U?
> 
>(Deleted)
>
>Having the memory physically contiguous is not enough. You also have to map 
>the physical region at the same base address in the guest to be able to 
>program the DMA correctly.
>
>Although, beware that device passthrough is not safe without IOMMU. Your 
>Android guest will be able to issue DMA request to any part of the RAM and 
>could interfere with DomU_Function.
>
>If you care about security, I would advice you to look at implementing new PV 
>drivers.
>
>> As you mentioned, if we want to solve the memory-contiguous problem on DomU, 
>> we have to hack through the domain-memory-allocation path right?
>> Could you tell us where is the location of DomU memory construction flow 
>> that we could do some modification?
>
>FIY, a thread has been started few months ago with some idea how to support 
>1:1 mapping for guest (see [1]).
>
>In outline, you first have to modify the memory allocation in the toolstack 
>(see populate_guest_memory in tools/libxc/xc_dom_arm.c) to handle the 
>contiguous size you want to support.
>
>You then need to find a way to get the physical address of the block from the 
>hypervisor and write them in the guest DT. This also means the guest layout 
>which is currently hardcoded (see
>xen/include/public/arch-arm.h) needs to be modified for your purpose.
>
>At that point, you may hit some problem with the Xen allocator because nothing 
>guarantee that you will be able to find enough space to allocate the memory 
>contiguously.
>
>> Really thanks for your help.
>
>Regards,
>
>[1]
>http://lists.xenproject.org/archives/html/xen-devel/2015-02/msg00570.html
>
>--
>Julien Grall

Thanks Julien

We decieded to create back/front-end drivers to solve contiguous-memory problem 
on Dom0.
Currently checking through the grant-table / front-backend driver / xenstore 
stuffs.

But I am wondering that will memory operation become bottleneck when using 
grant-table to transfer data since double of the memory operation will be 
needed (please correct me if I am wrong).
        EX : 1. DomU copys data to Grant Page; 2. Dom0 Copys data from Grant 
Page to target memroy.

Or, probably I could reserve a contiguous-memory (probably tens of MB), then 
use grant-table share these pages to DomU.
Then DomU could take these granted pages as contiguous memory?Or is there any 
better advice or probably the overhead could simply been ignored.

Thanks
Tom


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