Re: [PATCH] xen: arm: configure correct dom0_gnttab_start/size

[David Vrabel <david.vrabel@citrix.com>]

On 11/09/14 17:05, Ian Campbell wrote:
> On Thu, 2014-09-11 at 16:54 +0100, Andrew Cooper wrote:
>> Why it is hard coded at all? (x86 appears to manage fine.)  Without it
>> being variably-located, there will always be a risk of collisions like this.
> 
> For x86 PV (and shadow?) you can establish a mapping of the gnttab from
> a virtual address directly to the machine address without needing a p2m
> entry, via whichever hypercall it is which does that.
> 
> For HAP (and shadow?) you need a p2m mapping to get at the grant table,
> so somewhere has to be found in the physical address space where it can
> go.
> 
> In theory a guest kernel could try and find some io space which it isn't
> using but to help them out we give them a hint.
> 
> For x86 HVM the IOBAR of the platform device as a handy place to put the
> hint, for ARM domU we have a defined region in our address map which we
> use and communicate via device tree.
> 
> But for ARM dom0 the physical address space layout follows the host,
> figuring out a free bit of space is a bit tricky so each platform is
> currently required (except some don't and there is a broken default) to
> say where is a safe place to use (i.e. a person has to read the
> datasheet and figure it out).

Linux x86 PVH dom0 and domU uses ballooned pages and they're vmap()'d in
into a contiguous bit of virtual address space.

Perhaps something similar could be considered for ARM in the future?
This would be common code between ARM and x86 PVH.

David