hermit-sys: Network regression

[hermit-os]

hermit-sys: Network regression

[mkroening]

Bisecting nightly releases shows that the issue appears when upgrading RustyHermit from nightly-2021-03-22 to nightly-2021-03-23 as demonstrated in #134. I'll have to investigate, what happened in that night (rust-lang/rust@f826641...5d04957).

[mkroening]

Ah, indeed code related to network changed, in hermit-os/kernel@5f2d3b1...44eaa9f, but GitHub does not show the diff nicely.

Edit: Nevermind, I was just confused by the force-push in libhermit and git diff... I have no idea, what could be the cause of this issue. 🤔

[mkroening]

I found the cause. Mutable noalias has been enabled by default in rust-lang/rust@39ed643. Reverting to the old behavior via RUSTFLAGS="-Zmutable-noalias=no" works around the issue and network works again, but can't be considered a proper fix.

Mutable noalias has enabled further optimizations of the reads and writes in ComCfg::dev_features, even in debug mode. Making them volatile, as they should be, solves the issue. Inserting debugging prints of self.com_cfg.device_feature_select does too, making this a prime example of a Heisenbug.

I'll look into proper abstractions for volatile memory access, so we can prevent this from happening in other places as well.

[stlankes]

May be gnoliyil/fuchsia@914bdaf helps to understand the problem.

[mkroening]

Thanks for the Link!

I think, we are not facing a miscompilation here though. A fix for that specific issue is in place since nightly-2021-05-14 (rust-lang/rust#84958 (comment), we are on nightly-2021-05-19). Also, our issue also exists in debug.

In the long term we should make sure to make all memory-mapped I/O accesses volatile, but to get things working again as soon as possible, we might want to disable mutable-noalias until we have a proper fix. I'll create tracking issues and PRs.