Re: [PATCH V4 3/3] libxl/arm: Add handling of extended regions for DomU

[Oleksandr <olekstysh@xxxxxxxxx>]

On 30.09.21 01:52, Oleksandr Tyshchenko wrote:

Hi Stefano, Julien.

> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
> 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. As region 0 is reserved for grant table
> space (always present), the indexes for extended regions are 1...N.
> If extended regions could not be allocated for some reason,
> Xen doesn't fail and behaves as usual, so only inserts region 0.
>
> Please note the following limitations:
> - The extended region feature is only supported for 64-bit domain
>    currently.
> - The ACPI case is not covered.
>
> ***
>
> The algorithm to choose extended regions for non-direct mapped
> DomU is simpler in comparison with the algorithm for direct mapped
> Dom0. As we have a lot of unused space above 4GB, provide single
> 2MB-aligned region from the second RAM bank taking into the account
> the maximum supported guest address space size and the amount of
> memory assigned to the guest. The maximum size of the region is 128GB.
> The minimum size is 64MB.
>
> Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>
> Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
> Tested-by: Michal Orzel <michal.orzel@xxxxxxx>
I though a bit more on this and decided to make a patch more functional 
by trying to also allocate extended region below 4GB, I think we could 
do with it.
Actually if guest memory size is less than GUEST_RAM0_SIZE, we are able 
to provide unused space. I have tested with with various guest memory 
sizes and it worked fine. Also I decided to drop limit for maximum 
extended region size (128GB), we don't apply this limit in Dom0 and I 
don't see why we need it here, moreover the calculation became more 
obvious. I will drop all acks and send updated version. Are there any 
objections?


--
Regards,

Oleksandr Tyshchenko