Re: [Xen-devel] xen arm64 dom0 question

[Julien Grall <julien.grall@xxxxxxx>]

On 02/09/16 12:27, Peng Fan wrote:
> Hi Julien, Stefano
Hi Peng,

> On My ARM64 platform, there is 6GB memory.
> 0x80000000 - 0xfffffff: 2GB
> 0x880000000 - 0x9ffffffff: 4GB
>
> xen will alloc 1:1 mapping for Dom0 memory, so if I assign dom0_mem with a 
> bigger
> value, saying 2048MB or bigger. xen will alloc continus memory from higher 
> address
> space in the higer 4GB.
>
> But the SD controller in my ARM64 platform has a limitation that it can only
> access 32bit address space. So if Dom0 memory is at higher 4GB, SD controller
> can not work as expected, because it only works when the dma address is 32bit
> address.
>
> So, Can we assign a hole in lower 32bit address space for Dom0 guest physical
> memory, just as DomU? Dynamically find out a hole and size 128MB or bigger?
> Or do you have any ideas?
Looking at the allocation code (allocate_memory in 
arch/arm/domain_build.c), Xen is trying to allocate as much memory as 
possible below 4GB for 32-bit domain to accommodate non-LPAE kernel.
We haven't though about devices that can only handle 32-bit address at 
that time. I think replacing "lowmen = is_32bit_domain(d)" by "lowmem = 
true" should do the job.
Note that, a proper upstream patch would require to modify the 
description of the function and potentially kill lowmen (or gate it with 
a command line parameter?).
Regards,

--
Julien Grall

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