Support OCaml 5.2

[shym]

This PR brings support for OCaml 5.2 (5.2.0~beta2 at the moment).
It proposes to do so by revising substantially how the cross compiler is built and so, unfortunately, drops support for OCaml 4.x compilers.

This builds upon the great work that has been done in the following previous PRs:

• [WIP] OCaml 5.0 support #122
• Support of OCaml 5.0 (cleaned version) #124
• 500 cleaned update #129

and so I chose to branch it from #129’s branch, before the changes to the memory allocator (which are not necessary for OCaml 5.2), and rebased it all on main. All the comments in those PRs have been taken into account.

This PR is marked as draft as it should be merged only when OCaml 5.2 is released but it can be tested in a 5.2.0~beta2 switch.

Main changes

To support OCaml 5.2, this PR extends nolibc as necessary to build the runtime. Most of those extensions are inherited from those previous PRs.
Apart from that, the main changes of this PR are as follows.

Patches to the compiler sources

This PR proposes to change from the scheme of modifying the OCaml build system by seds and echos and instead brings a series of (version-specific) patches in patches/ that are applied on the sources when they are fetched.

There two main reasons for this change.

• The first is that those patches (except the last one) were written in a way to improve the capability of the compiler build system to generate cross compilers, not only for Solo5, with the intention of upstreaming them. This would ease a lot the maintenance for OCaml/Solo5 (and hopefully similar projects). As we are quite advanced in the release cycle for 5.2 and so those patches would not get merged into that release, the idea is to take the time to test whether that’s really what we would require for Solo5 and propose them upstream when a strong case can be made for them.
• The second reason is that having those modifications as full patches (they are git format-patch patches, so they contain a full explanation message for each change) makes them easier to review. (In my review of the previous PRs, the modified Makefile of the compiler was broken so make was generating warnings, even if it was still able to build the compiler; using actual patches makes it easier to make sure what modified build system we will actually rely on)

One interesting outcome of this deep revision of the compiler build system is the fact that the .opt versions of the compilers are built, which should give better performance, in particular to build large unikernels.

Open question for those patches: I kept git format-patch default of using a binary diff (for configure, as it is declared binary in the compiler) as what is useful to review is the changes to configure sources that are always bundled in the same patch. git format-patch -a patches would be bigger (twice the size, with the current patch series) but they might be preferred for other reasons.

A specific toolchain

The package now creates and installs a {aarch64,x86_64}-solo5-ocaml-* toolchain, in a similar fashion to what Solo5 does. The main reason for creating that toolchain in the first place was to avoid baking the build-time directories containing nolibc and openlibm into the generated OCaml compilers. Two versions of the toolchain are generated: one with the build-time directories, which is added to PATH during the build of the compiler; one with the final destination directories, which is installed in bin.
Besides making it possible to change the -I and -L between build-time and final versions, I further used this idea to:

• store in the cc all its other required options (such as -include _solo5/overrides.h), since this machinery is rather well-placed there,
• wrap all the binutils similarly; as an end-user, I find it handier to have all the prefixed tools available rather than having to remember for each of them which tool I should call (aarch64-solo5-none...*? aarch64-linux-gnu-*? ...). This fits in nicely with OCaml configure’s toolchain discovery so that stating the --target is enough to define all the tools of the toolchain.

Installation

This PR proposes to delegate the actual installation to OPAM (or opam-installer) by creating a ocaml-solo5.install file rather than invoking the install command itself. The initial reason for this choice is that it is fairly easy to bend OCaml’s make install into generating the list of all the files it could install and then tell OPAM to install them only if they were actually built. So we can skip building parts of the compiler (ocamllex, etc.) just to keep make install happy when they don’t make sense.

This comes with the natural consequence that it follows OPAM’s conventions when installing files, so the compiler is installed in %{prefix}%/lib/ocaml-solo5 instead of %{prefix}%/solo5-sysroot (I find following the OPAM convention a rather good idea anyhow). If we relax the name of the findlib toolchain, it becomes possible to install simultaneously ocaml-solo5 and ocaml-solo5-cross-aarch64 (in my testing, I didn’t switch between the two as much as I would have if they had been co-installable), since the only common file between those two packages is the findlib’s solo5.conf.

Smaller changes

Quite a few commits are clean-ups for things that were natural to fix while working on this (such as ensuring that make will rebuild nolibc and openlibm libraries iff that should be done). I feel that those small changes will make maintenance smoother.

Another change from previous PRs is the choice to drop stdatomic.h from nolibc and go with the one of the underlying compiler, to make sure the C compiler will compile C atomics as it is intended to do. This only needs a small patch to Solo5 for FreeBSD, to pull it along with the other freestanding headers when installing the Solo5 toolchain.

Testing

This has been tested on:

• Debian x86_64 with gcc, target aarch64, building (and briefly running) the dns-resolver hvt mirage unikernel,
• Debian x86_64 with gcc, target x86_64, at least running the internal tests,
• FreeBSD 14 x86_64 with clang, target x86_64, running the internal tests; this needs a small patch on Solo5, to access the stdatomic.h header, see Fetch stdatomic.h from system on FreeBSD Solo5/solo5#574
• OpenBSD 7.3 x86_64 with clang, target x86_64, running the internal tests
• OpenBSD 7.5 x86_64 with clang: Solo5 is currently broken there, Initial support for OpenBSD 7.5 Solo5/solo5#573 has been opened there with some steps towards supporting the latest OpenBSD.

Open question

I have been wondering about how interrupts are set up in Solo5 (without taking the time to really dig, in particular because the precise set up depends on the binding), because the way the OCaml 5 runtime sets up its stacks is really different from OCaml 4.
In particular, the OCaml 5 runtime assumes that only OCaml code should ever be triggered on a fiber stack. Is Solo5 already currently set up so that it can be guaranteed?
Commit “WIP Disable the x86_64 red zone” is strongly related to that question, on a slightly different angle.

To do before merging

• “TMP” commits should be updated to OCaml 5.2
• the questions in the “WIP” commits should be decided
• README should be updated

[dinosaure]

I have been wondering about how interrupts are set up in Solo5 (without taking the time to really dig, in particular because the precise set up depends on the binding), because the way the OCaml 5 runtime sets up its stacks is really different from OCaml 4.
In particular, the OCaml 5 runtime assumes that only OCaml code should ever be triggered on a fiber stack. Is Solo5 already currently set up so that it can be guaranteed?
Commit “WIP Disable the x86_64 red zone” is strongly related to that question, on a slightly different angle.

To contextualize the Solo5 side, I found:

• this commit which introduces -mno-red-zone for bindings: Solo5/solo5@2f1cae3
• this issue (still open): Remove -m-no-red-zone on AMD64 Solo5/solo5#292

[shym]

Thank you for the pointers. How to properly setup interrupt handling on x86_64 is quite some rabbit hole! To sum up what I learnt:

• switching stack automatically on interrupt with no priviledge change is possible on x86_64 with IST,
• setting up the hardware correctly for a real bare-metal kernel is terribly tricky (1),
• but Solo5 makes a very limited and controlled use of interrupts, so that it is (hopefully) bypassing the reentrance problem,
• if there is nevertheless a reentrance problem for interrupts, the problem is happening on the interrupt stacks, there should not be problems on the stacks as seen by the OCaml runtime.

So I’ve dropped the commit disabling the red zone.
(And pinned the branch of Solo5 to fix FreeBSD support)

[hannesm]

Dear @shym, thanks for rebasing. Would you mind to mark it as ready for review? (Unless you still have some things you want to address first -- but in the meeting today it sounded like this is ready from your side)

[shym]

I’ve rebased on main (cc @palainp) and added a few small fixes (left explicitly as fixup!... commits for now, to make it easier to resync for reviews that were already under way).
It was marked as draft (maybe wrongly, I see) as it should not be merged before OCaml 5.2 is out, but it is ready for review as is.

[shym]

I notice only now that something doesn’t look just right in the implementation of mmap with a non-NULL argument (cc @palainp).

[shym]

I dug deeper into the usage of mmap in the OCaml code base and got reassured: if I understand correctly, mmap is used with a (non-NULL) address only with MAP_FIXED to commit and decommit memory (set its access to rw- or ---) that has been previously reserved. So we should be fine just ignoring those mmaps, since we don’t handle memory protection. 😅
I added commit Handle mmap(addr..., MAP_FIXED, ...) uses to change the behaviour accordingly.
I would have been reassured about my understanding if I had managed to find an example that follows that code path, but the small programs I ran didn’t commit/decommit memory.

Finally, I also updated the PR to OCaml 5.2 RC1, released on Friday.

[palainp]

Thank you, sorry I've still not managed to test :(
But now that 5.2 is out, I guess it's ok to update this PR against the latest release :)

[shym]

I’ve updated the PR to OCaml 5.2.0 as it was released last week.

Note: I’ve reverted to depend on the ocaml package (as it was before) rather than a precise dependency on a compiler package (ocaml-base-compiler or ocaml-variants, which was more relevant for pre-release versions of the compiler). But the build will fail later if the compiler in PATH is not exactly 5.2.0 because I left a strict test of version equality in the configure script. I don’t think it’s worth trying to relax that constraint (ie allow to build the 5.2.0 cross-compiler in a 5.2.0~rc1 switch) so I feel more confident keeping it that way.

[palainp]

Thank you @shym for that update! I can confirm that this PR compiles and qubes-mirage-firewall compiles & runs as with 4.14.2 (first fast.com tests show around 10-15% more bandwidth now but this wasn't conducted with a strict evaluation protocol :) ). I'll also try with longer runs.

[dinosaure]

On my side, I approve this PR. It took time for several people and I think the review of the various maintainers and authors has been done. Thank you @shym for taking over and considering what may have blocked us in our previous PRs, I consider that this one can be merged 👍. /cc @mirage/core

[palainp]

I also approve this PR, but as it breaks the supported compilers versions, and I think we also need to keep a 4.4 branch corresponding to the current head before merging :)

[shym]

I’ve just updated the patches to the compiler so that it contains the patches that are already integrated upstream and in the version integrated upstream: it will make it much easier to re-synchronize when updating the patches for the next version.
I’ve also been working on the other patches, to generalize them to cover more cross-compiler use cases, but I judged it best not to update them that late in the review process and considering that they are not yet upstreamed.
If you want to have a look, it might be easier to compare the branches:

• before that update: solo5,
• after that update: solo5-v1.5,
• the currently cooking version of the patches, if you will adventurous: solo5-v2.