Re: [PATCH v2 1/4] x86: provide an inverted Kconfig control for shim-exclusive mode

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Jan 21, 2025 at 09:13:38AM +0100, Jan Beulich wrote:
> On 21.01.2025 00:27, Stefano Stabellini wrote:
> > On Mon, 20 Jan 2025, Jan Beulich wrote:
> >> On 18.01.2025 00:41, Andrew Cooper wrote:
> >>> On 17/01/2025 10:43 pm, Stefano Stabellini wrote:
> >>>> On Fri, 17 Jan 2025, Jan Beulich wrote:
> >>>>> On 17.01.2025 13:24, Alejandro Vallejo wrote:
> >>>>>> On Fri Jan 17, 2025 at 10:31 AM GMT, Roger Pau Monné wrote:
> >>>>>>> On Thu, Jan 16, 2025 at 04:31:46PM -0800, Stefano Stabellini wrote:
> >>>>>>>> On Wed, 1 Mar 2023, Jan Beulich wrote:
> >>>>>>>>> While we want certain things turned off in shim-exclusive mode, doing
> >>>>>>>>> so via "depends on !PV_SHIM_EXCLUSIVE" badly affects allyesconfig: Since
> >>>>>>>>> that will turn on PV_SHIM_EXCLUSIVE, other options will be turned off as
> >>>>>>>>> a result. Yet allyesconfig wants to enable as much of the functionality
> >>>>>>>>> as possible.
> >>>>>>>>>
> >>>>>>>>> Retain PV_SHIM_EXCLUSIVE as a prompt-less option such that first of all
> >>>>>>>>> C code using it can remain as is. This isn't just for less code churn,
> >>>>>>>>> but also because I think that symbol is more logical to use in many
> >>>>>>>>> (all?) places.
> >>>>>>>>>
> >>>>>>>>> Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> >>>>>>>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >>>>>>>>>
> >>>>>>>>> ---
> >>>>>>>>> The new Kconfig control's name is up for improvement suggestions, but I
> >>>>>>>>> think it's already better than the originally thought of
> >>>>>>>>> FULL_HYPERVISOR.
> >>>>>>>> I think the approach in general is OK, maybe we can improve the naming
> >>>>>>>> further. What about one of the following?
> >>>>>>>>
> >>>>>>>> NO_PV_SHIM_EXCLUSIVE
> >>>>>>>> PV_SHIM_NOT_EXCLUSIVE
> >>>>>>> IMO negated options are confusing, and if possible I think we should
> >>>>>>> avoid using them unless strictly necessary.
> >>>>>> The problem is that the option is negative in nature. It's asking for
> >>>>>> hypervisor without _something_. I do agree with Stefano that shim would be
> >>>>>> better in the name. Otherwise readers are forced to play divination tricks.
> >>>>>>
> >>>>>> How about something like:
> >>>>>>
> >>>>>>     SHIMLESS_HYPERVISOR
> >>>>>>
> >>>>>> That's arguably not negated, preserves "shim" in the name and has the correct
> >>>>>> polarity for allyesconfig to yield the right thing.
> >>>>> Except that a hypervisor with this option enabled isn't shim-less, but permits
> >>>>> working in shim as well as in non-shim mode.
> >>>> First, let's recognize that we have two opposing requirements. One
> >>>> requirement is to make it as easy as possible to configure for the user.
> >>>> Ideally without using negative CONFIG options, such as
> >>>> NO_PV_SHIM_EXCLUSIVE. From the user point of view, honestly,
> >>>> PV_SHIM_EXCLUSIVE is a pretty good name. Better than all of the others,
> >>>> I think.
> >>>>
> >>>> On the other hand, we have the requirement that we don't want
> >>>> allyesconfig to end up disabling a bunch of CONFIG options. Now this
> >>>> requirement can be satisfied easily by adding something like
> >>>> NO_PV_SHIM_EXCLUSIVE. However, it would go somewhat against the previous
> >>>> requirement.
> >>>>
> >>>> So we need a compromise, something that doesn't end up disabling other
> >>>> CONFIG options, to make allyesconfig happy, but also not too confusing
> >>>> for the user (which is a matter of personal opinion).
> >>>>
> >>>> In short, expect that people will have different opinions on this and
> >>>> will find different compromises better or worse. For one, I prefer to
> >>>> compromise on "no negative CONFIG options" and use
> >>>> PV_SHIM_NOT_EXCLUSIVE. Because it serves the allyesconfig goal, and
> >>>> while it is not as clear as PV_SHIM_EXCLUSIVE, is still better than a
> >>>> completely generic FULL_HYPERVISOR option, which means nothing to me.
> >>>>
> >>>> I cannot see a way to have a good and clear non-negated CONFIG option,
> >>>> and also satisfy the allyesconfig requirement. So I prefer to compromise
> >>>> on the "non-negated" part.
> >>>
> >>> Debating the naming is missing the point.
> >>>
> >>>
> >>> The problem here is the wish to have PV_SHIM_EXCLUSIVE behave in a way
> >>> that Kconfig is not capable of expressing.  Specifically, what is broken
> >>> is having "lower level" options inhibit unrelated "higher level" options
> >>> when the graph gets rescanned[1].  That's why we're in the laughable
> >>> position of `make allyesconfig` turning off CONFIG_HVM.
> >>>
> >>> Jan, you want "echo PV_SHIM_EXCLUSIVE=y >> .config && make" to mean
> >>> "reset me back to a PV Shim".
> >>
> >> Isn't this an independent goal? Or is this a statement on what you see
> >> my change (kind of) doing, indicating you consider this wrong?
> > 
> > The way I understood it, it is the latter
> > 
> > 
> >>> Kconfig spells this "make $foo_defconfig" for an appropriately given foo.
> >>>
> >>>
> >>> There should be:
> >>>
> >>> 1) an option called PV_SHIM_EXCLUSIVE which does *nothing* other than
> >>> making the boolean be a compile time constant.
> >>
> >> I fear it remains unclear to me what exactly you mean here. It feels like
> >> you may be suggesting that all other uses of PV_SHIM_EXCLUSIVE should be
> >> dropped, without replacement. That seems wrong to me, though. In
> >> particular I'm of the opinion that besides using "make pvshim_defconfig"
> >> people ought to also have the option to properly configure a shim build
> >> from scratch (or from any random .config they hold in hands), requiring
> >> respective "depends on" and/or "select" / "imply" to be in place.
> > 
> > That should be done with "make pvshim_defconfig"
> 
> Why? Specifically, why should people use only one entirely nailed down
> configuration for a shim. Like a "normal" hypervisor, there are aspects
> which very well can be left to the person doing the configuration.

But nothing prevents a user from starting from a shim defconfig, and
then tweaking it as desired:

$ make pvshim_defconfig
$ make menuconfig

Or there's something I'm missing here?

> >> Or else they may end up with a lot of dead code. (Just consider e.g.
> >> HVM=n: We also don't permit HVM-only stuff to be enabled in that case,
> >> as any of that would again be dead code then.)
> > 
> > It will always be possible to come up with poor configurations. I do not
> > believe it is necessarily our responsibility to go out of our way to
> > prevent them.
> 
> Well - if so, why would we do this in some cases but not in others?
> You may recall that I'm a proponent of consistency and predictability.
> 
> >>> 2) a pvshim_defconfig target which expresses what a pvshim ought to look
> >>> like.
> >>
> >> We have this file already. I consider it debatable though whether this file
> >> should really force PV_SHIM_EXCLUSIVE=y. People may read "pvshim" in the
> >> name as either "works just as shim" or "can also work as shim".
> > 
> > If I understood it right, I like Andrew's suggestion. He is suggesting
> > to do the following:
> > 
> > - turning PV_SHIM_EXCLUSIVE into something that does nothing
> 
> FTAOD - you mean Kconfig-wise? Andrew clearly didn't say "nothing", but
> "nothing other than making the boolean be a compile time constant".

Won't making the boolean a compile time constant would also result in
DCO kicking in and removing a fair amount of code?  So even if you
have enabled everything in Kconfig, the resulting hypervisor would
only be suitable to be used as a shim?

Thanks, Roger.