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Re: [Xen-devel] [PATCH] xen/arm: Allocate memory for dom0 from the bottom with the 1:1 Workaround

On 22 October 2013 12:26, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> On Tue, 2013-10-22 at 11:51 +0100, Julien Grall wrote:
>> On Linux, the option CONFIG_ARM_PATCH_PHYS_VIRT (by default enabled) allows
>> the Kernel to be loaded anywhere (or nearly) by patching the translation
>> pv<->virt at boot time.
>> The current solution in Linux assuming that the delta physical address -
>> virtual address is always negative. A positive delta will destroy all the
>> optimisation to modify only a part of the translation instruction (add/sub).
>> By default, Xen is allocating memory from the top of memory and then
>> goes down. To avoid booting issue with Linux, we must allocate memory
>> from the bottom (ie starting from 0).
>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> OOI how low does the memory get allocated in practice? We deliberately
> load Xen and place xenheap up high so I guess for dom0 we can't get
> memory from pretty low down?

It depends where U-boot has loaded the different modules (kernel,
initrd, device tree,...).
If they are not loaded at the bottom of the RAM, Xen is able to use
the first address of the first bank.

Julien Grall

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