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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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@xxxxxxxxxx>
> >>>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >>>>>>>
> >>>>>>> ---
> >>>>>>> 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"


> 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.


> > 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
- adding "make pvshim_defconfig"

So that:

- people use "make pvshim_defconfig" to get what today is enabled by
  PV_SHIM_EXCLUSIVE
- but "make allyesconfig" doesn't end up disabling things
- the Kconfig menu makes sense because PV_SHIM_EXCLUSIVE goes away and
  it is not replaced by anything

If I got it right, I am in favor.