Re: [PATCH v2 03/14] x86/shstk: Introduce Supervisor Shadow Stack support

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Tue, Jun 02, 2020 at 02:41:30PM +0200, Jan Beulich wrote:
> On 02.06.2020 14:26, Anthony PERARD wrote:
> > On Tue, Jun 02, 2020 at 02:06:11PM +0200, Jan Beulich wrote:
> >> I don't recall; perhaps I was in another parallel session? If it's
> >> the one with notes at
> >> https://lists.xenproject.org/archives/html/xen-devel/2019-07/msg00786.html
> >> then a remark close to the top suggests I was there, but there's no
> >> sign of this aspect having got discussed. There is, among the issues
> >> listed, "Xen build re-evaluates compiler support for every translation
> >> unit", but that's only remotely related.
> > 
> > What is a "translation unit"? What would that have to do with Kconfig?
> 
> A translation unit is the collective input from all source and
> header files seen by the compiler during the processing of one
> top level input file's worth of compilation.

Thanks. Now it feels like the issue listed in the design session note
isn't exactly correct, compiler didn't used to be evaluated once per CU,
but only once per Makefile loaded/executed.

> The connection to
> Kconfig here is that by switching to it, the compiler flags
> don't get re-constructed once per CU. Of course doing it via
> Kconfig is not the only possible solution to the problem (as
> can be seen from the patch that I had intermediately submitted
> to get at least the HAVE_AS_* out of that set), but for us the
> change in behavior is one intended (side-)effect of the switch
> to newer Kconfig.

Well, with a recent patch of mine, tool chain support should already be
only evaluated only once in the root Makefile. So I do also think that
we don't need to move all of them to Kconfig.

So advantage of evaluating some compiler flags in Kconfig is that we
can have other CONFIG_* depends on those flags; the inconvenient is to
figure out when do we need to re-evaluate the .config.

-- 
Anthony PERARD