Re: [Xen-devel] Xen on ARM: "Failed to allocate contiguous memory for dom0" with dom0_mem greater than 256MB

[Arianna Avanzini <avanzini.arianna@xxxxxxxxx>]

On 01/24/2014 11:14 AM, Ian Campbell wrote:
> On Fri, 2014-01-24 at 11:00 +0100, Arianna Avanzini wrote:
>> I noticed that Xen 4.4 fails to boot the dom0 if more than 256MB of RAM are
>> assigned to it by providing the dom0_mem boot option. The error message 
>> produced
>> during the boot process is the following.
> 
> This is an unfortunate side effect of the use of the 1:1 mapping. The
> threads:
>         "Master not working on Allwinner A20"
>         "create multiple banks for dom0 in 1:1 mapping if necessary"
> Have some more details, but in short the allocation of dom0 memory needs
> to be done in a single chunk and because of the way the Xen allocator
> works it will also be aligned to its own size -- this creates some
> limitations on the size of the region vs what memory is free at start of
> day
> 
> I'm afraid this probably won't be solved for Xen 4.4, since the change
> is likely to be rather intrusive.
> 

I see, thank you for the explanation.


> As a workaround you could try changing the load addresses of Xen and the
> kernel, dtb etc used by u-boot to pack them towards the top of RAM. This
> *should* result in the entire lower half of RAM being available which
> will make it more likely to achieve the necessary alignment constraints
> for a dom0 taking up to half of the system RAM. I've not actually tried
> this but I'd recommend trying the following addresses from the top of
> RAM:
>       -2M: Leave free for Xen to relocate to
>       -4M: dom0 kernel
>       -6M: DTB
>       -8M: Initial Xen load address
> 
> If you have an initrd then put it between dom0 kernel and dtb and bump
> everything else down, likewise if anything is bigger than 2M then round
> up to 2M and push everything down to accommodate it.
> 
> Also consider that for a Xen system it is common to only give dom0 a
> fairly small fraction of the system RAM in order to leave as much as
> possible for guest domains. What target amount of dom0 memory are you
> aiming for? 256M or even 128M is probably plenty if you are going to run
> a few domains.
> 

So far, 256MB of RAM are actually enough for a dom0 in my use-case. Thank you
again for the info and hints, and for taking the time to test both the failing
scenario and the workaround.



> Ian.
> 


-- 
/*
 * Arianna Avanzini
 * avanzini.arianna@xxxxxxxxx
 * 73628@xxxxxxxxxxxxxxxxxxx
 */

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