Re: [Xen-devel] [PATCH v2 08/10] arm: add a small kconfig for Renesas RCar H3

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 22/05/2018 22:00, Stefano Stabellini wrote:
> On Tue, 22 May 2018, Julien Grall wrote:
> > Hi,
> >
> > On 05/22/2018 01:53 AM, Stefano Stabellini wrote:
> > > This is a reference tiny kconfig for Renesas RCar.  In terms of
> > > schedulers, it selects credit and NULL only.  It enables all the ARM64
> > > errata.
> > It still does not feel right that you select only credit and NULL. Why not
> > credit2 and NULL? Or other combination.
> We have to pick a combination of options for certifications and this is
> the one I am proposing: we need the null scheduler for latency sensitive
> mission critical VMs and we need credit (the default today) for the
> others.
>
> I am happy to discuss the pros and cons of other combinations.
The .config is very subjective and I don't think we can possible cater 
everyone here. For instance, someone might want a different .config for 
that board with credit2... If someone ask for adding this option, what 
would you answer to them? You can't turn them down because this .config 
is for certification.
But I don't think your solution is the right way to go. See more below 
for some suggestion.
>   
> > > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > CC: artem_mygaiev@xxxxxxxx
> > > CC: volodymyr_babchuk@xxxxxxxx
> > >
> > > ---
> > >
> > > This patch is untested on Renesas RCar, please test!
> > > Also, I am not sure whether some of the errata workarounds can be
> > > disabled on the RCar.
> > >
> > > Changes in v2:
> > > - rename to rcar3
> > > - only add required symbols, let the defauls take care of the rest
> > I am not sure what you mean here. Your .config below seems contains all the
> > options including the non-selected one.
> >
> > Also, this still not solving the problem raised by Andrew regarding keep them
> > updated.
> It does not have all the options: it only contains the non-default
> options as per Juergen's suggestion:
>
> https://marc.info/?l=xen-devel&m=152419926530183
Are you sure? For instance, CONFIG_ACPI is turned off by default but 
still present in the .config. Maybe I am missing something.
> > It might be easier to maintain if we provide a per platform config option (e.g
> > CONFIG_RCAR3) that will select driver for that specific board.
> >
> > The user is then free to select other components (e.g scheduler...). So you
> > don't impose memaccess disabled, NULL scheduler...
> >
> > (Thank you Andrii for the suggestion!)
> This is a good idea, it would be great to have CONFIG_RCAR3, but it does
> not take away the need for this kconfig. CONFIG_RCAR3 and rcar3.config
> are orthogonal, let me explain.
>
> Let's say that we have a CONFIG_RCAR3 that selects everything needed for
> the Rcar3, such as:
>
> NR_CPUS, SCIF
>
> and deselects:
>
> ACPI, GICV3, the other UARTs, ARM_SMMU.
>
> We still need a reference kconfig with other not platform specific
> options, for instance:
>
> SCHED_NULL
>
> For two reasons:
> 1) we need a reference kconfig for certifications, it has to include the
>     choice of schedulers, debug options, etc, which are not Rcar3 specific
As you said it is not Rcar3 specific. So this would have to be 
duplicated on each .config (QEMU...).
It really feels like we want some sort of partial .config (similar to 
what Linux has [1]) that will select non-specific platform option. We 
could have a tiny one, certifiable, "all", server...
> 2) as per previous discussions, we need a set of pre-canned kconfigs to
>     establish what we security support
>
> rcar3.config is meant to address these two points. CONFIG_RCAR3 would
> not take away the need for rcar3.config, but it would make rcar3.config
> shorter and easier to maintain.
>
> CONFIG_RCAR3 was not on my roadmap but I'll see what I can do. Maybe it
> is best if I do the work for QEMU only (both CONFIG_QEMU and
> qemu.config) and leave the Renesas work (both CONFIG_RCAR3 and
> rcar3.config) to EPAM. I cannot test it anyway.
Cheers,

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/configs
--
Julien Grall

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