Re: [RFC PATCH 0/3] Add handling of extended regions (safe ranges) on Arm (Was "xen/memory: Introduce a hypercall to provide unallocated space")

[Oleksandr <olekstysh@xxxxxxxxx>]

Hello all


On 10.09.21 02:20, Oleksandr wrote:
> Hello all
>
> On 07.09.21 20:09, Oleksandr Tyshchenko wrote:
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> >
> > You can find an initial discussion at [1].
> >
> > The extended region (safe range) is a region of guest physical
> > address space which is unused and could be safely used to create
> > grant/foreign mappings instead of wasting real RAM pages from
> > the domain memory for establishing these mappings.
> >
> > The extended regions are chosen at the domain creation time and
> > advertised to it via "reg" property under hypervisor node in
> > the guest device-tree.
> >
> > The extended regions are calculated differently for direct mapped
> > Dom0 (with and without IOMMU) and non-direct mapped DomUs.
> >
> > Please note the following limitations:
> > - The extended region feature is only supported for 64-bit domain.
> > - The ACPI case is not covered.
> >
> > Also please note that we haven't figured out yet how to properly
> > extend the Xen hypervisor device-tree bindings on Arm (either via new
> > compatible or via new property). I decided to go with new property
> > for now, but this can be changed. This uncertainty is the main reason
> > why this series is marked as RFC.
> Sorry, I messed up the device tree binding's purpose here.
>
> New DT property "extended-region" (to inform guest about the presence 
> of additional regions in "reg" property) is not really needed. Guest 
> can simply infer that from the number of regions
> in "reg" property (region 0 - reserved for grant table space, regions 
> 1...n - extended regions).
> Instead, new compatible/property will be needed (but only after this 
> patch [1] or alternative goes in) to indicate that "region 0 is safe 
> to use". Until this patch is merged it is
> not safe to use extended regions for the grant table space.
>
> Thanks to Julien for clarifying these bits.
>
> I am going to remove the advertisement of unneeded "extended-region" 
> property in the code and send new version soon.
I have just pushed new version [1]. Please ignore this one. Sorry for 
the inconvenience.
[1] 
https://lore.kernel.org/xen-devel/1631297924-8658-1-git-send-email-olekstysh@xxxxxxxxx/

--
Regards,

Oleksandr Tyshchenko