Re: [GIT PULL] Second batch of KVM changes for Linux 5.6-rc4 (or rc5)

Linus Torvalds [off-list ref] writes:

On Sun, Mar 1, 2020 at 1:03 PM Paolo Bonzini [off-list ref] wrote:

quoted

Paolo Bonzini (4):
      KVM: allow disabling -Werror

Honestly, this is just badly done.

You've basically made it enable -Werror only for very random
configurations - and apparently the one you test.

Doing things like COMPILE_TEST disables it, but so does not having
EXPERT enabled.

So it looks entirely ad-hoc and makes very little sense. At least the
"with KASAN, disable this" part makes sense, since that's a known
source or warnings. But everything else looks very random.

I've merged this, but I wonder why you couldn't just do what I
suggested originally?

Seriously, if you script your build tests, and don't even look at the
results, then you might as well use

   make KCFLAGS=-Werror

instead of having this kind of completely random option that has
almost no logic to it at all.

And if you depend entirely on random build infrastructure like the
0day bot etc, this likely _is_ going to break when it starts using a
new gcc version, or when it starts testing using clang, or whatever.
So then we end up with another odd random situation where now kvm (and
only kvm) will fail those builds just because they are automated.

Yes, as I said in that original thread, I'd love to do -Werror in
general, at which point it wouldn't be some random ad-hoc kvm special
case for some random option. But the "now it causes problems for
random compiler versions" is a real issue again - but at least it
wouldn't be a random kernel subsystem that happens to trigger it, it
would be a _generic_ issue, and we'd have everybody involved when a
compiler change introduces a new warning.

I've pulled this for now, but I really think it's a horrible hack, and
it's just done entirely wrong.

Adding the powerpc people, since they have more history with their
somewhat less hacky one. Except that one automatically gets disabled
by "make allmodconfig" and friends, which is also kind of pointless.

Michael, what tends to be the triggers for people using
PPC_DISABLE_WERROR? Do you have reports for it?

My memory is that we have had very few reports of it actually causing
problems. But I don't have hard data to back that up.

It has tripped up the Clang folks, but that's partly because they're
building clang HEAD, and also because ~zero powerpc kernel developers
are building regularly with clang. I'm trying to fix the latter ...


The thing that makes me disable -Werror (enable PPC_DISABLE_WERROR) most
often is bisecting back to before fixes for my current compiler were
merged.

For example with GCC 8 if you go back before ~4.18 you hit the warning
fixed by bee20031772a ("disable -Wattribute-alias warning for
SYSCALL_DEFINEx()").

And then building with GCC head sometimes requires disabling -Werror
because of some new warning, sometimes valid sometimes not.

I think we could mostly avoid those problems by having the option only
on by default for known compiler versions.

eg:

config WERROR
	bool "Build with -Werror"
	default CC_IS_GCC && (GCC_VERSION >= 70000 && GCC_VERSION <= 90000)

And we could bump the upper version up once each new GCC version has had
any problems ironed out.

Could we have a _generic_ option that just gets enabled by default,
except it gets disabled by _known_ issues (like KASAN).

Right now I don't think we could have a generic option that's enabled by
default, there's too many warnings floating around on minor arches and
in odd configurations.

But we could have a generic option that signifies the desire to build
with -Werror where possible, and then each arch/subsystem/etc could use
that config option to enable -Werror in stages.

Then after a release or three we could change the option to globally
enable -Werror and opt-out any areas that are still problematic.

It's also possible to use -Wno-error to turn certain warnings back into
warnings even when -Werror is set, so that's another way we could
incrementally attack the problem.


It'd also be nice if we could do:

 $ make WERROR=0

Or something similarly obvious to turn off the WERROR option. That way
users don't even have to edit their .config manually, they just rerun
make with WERROR=0 and it works.

Being disabled for "make allmodconfig" is kind of against one of the
_points_ of "the build should be warning-free".

True, it was just the conservative choice to disable it for allmod/yes.
We should probably revisit that these days.

cheers