Re: [XEN][RFC PATCH v2 10/12] tools/libs/ctrl: Implement new xc interfaces for dt overlay

[Anthony PERARD <anthony.perard@citrix.com>]

On Thu, Nov 11, 2021 at 11:46:36AM -0800, Vikram Garhwal wrote:
> On Thu, Nov 11, 2021 at 04:54:25PM +0000, Anthony PERARD wrote:
> Hi Anthony,
> > On Mon, Nov 08, 2021 at 11:02:25PM -0800, Vikram Garhwal wrote:
> > > +SRCS-$(CONFIG_OVERLAY_DTB) += xc_overlay.c
> > 
> > So, this patch seems to introduce the use of CONFIG_OVERLAY_DTB, is
> > there a reason why the new functionality can't be always builtin?
> > 
> Above, if you meant removing "CONFIG_OEVRLAY_DTB" then here is my answer:
> This feature is supported by ARM based FPGA devices only so there were a few
> comments on v1 series to keep the code inside a config only. Now, for the tool
> side also I kept the CONFIG_OVERLAY_DTB to align the xen-tools with Xen.
> 
> Although, now i saw your comments on patch 10 regarding  "always provide
> libxl_dt_overlay() but which would return ENOSYS when libxl is built without
> CONFIG_OVERLAY_DTB". That seems better approach here for all three xen-tool
> patches.
> 
> Initially, i was not sure what to do here that's why i wrote a question in the
> cover letter about this.
> 
> Also, do you know how to enable this config via menuconfig when building the Xen
> tools? I know how to enable for Xen but not sure about tools.

It isn't possible to use the configuration of the hypervisor to build
the tool. We use autoconf (configure.ac, ...) to configure the tools but
I don't think in this case that having CONFIG_OVERLAY_DTB for the tools
is the right thing to do. In the tools, we don't really have a way to
select functionalities available in the different libraries, or it is
mostly based on the architecture or operating system or available
system libraries.

Contrary to the hypervisor side, the added code in the libraries is
mostly glue which calls the hypervisor, so their isn't really a need to
avoid building it. If the functionality isn't available in the
hypervisor, it should return an error and the library can deal with that
error.

You might want to limit the build to Arm, but I don't know if that
"overlay dtb" thing is really Arm specific (even though device trees are
mostly use on Arm I guess).

-- 
Anthony PERARD