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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 5 Oct 2021, Oleksandr wrote:
> > 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?

I am OK with it; it looks like you made good improvements. One caveat is
that I volunteer to review again no problem, but we'll need a new ack
from Ian Jackson to commit.