Re: [MirageOS-devel] Mini-OS on ARM status

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 6 May 2014, at 10:41, Thomas Leonard <talex5@xxxxxxxxx> wrote:
> On 6 May 2014 10:19, Anil Madhavapeddy <anil@xxxxxxxxxx> wrote:
>> 
>> We pretty much use a static stack in x86_64 as well, if I remember right,
>> and size it big enough that we don't blow past it casually.  Doing that
>> initially on ARM to get things going would be fine too.  I like the idea
>> of not requiring virtual memory to be used unless absolutely necessary,
>> since it avoids the unpredictability of demand paging.
>> 
>> We may need it for grant tables though...?
> 
> The FDT suggests an address for the grant table, and I currently just use
> XENMAPSPACE_grant_table so that Xen makes it appear there. I'm
> assuming that Xen's suggested address won't overlap any of the other
> addresses it passes us (all these addresses are hard-coded in Xen in
> arch-arm.h).
> 
> ARM has two stages to each address translation: VM virtual address to
> VM physical address (this mapping is managed by the VM), and then VM
> physical to actual physical address (controlled by Xen).
> 
> The guest's MMU is enabled (otherwise hypercalls don't work properly);
> it's just configured with a 1:1 mapping currently.

Thanks for clarifying -- that all makes sense to me now.  So we could
map a large VA area for a stack and demand map some spare pages in
there, but at the cost of losing the simple 1:1 mapping.  Just fixing
a static stack of a few megs seems the simplest approach for now.
Not much of our code requires a deep stack (or the x86_64 MiniOS would
keel over too).

-anil

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