Hi Alice, On Tue, Jun 04, 2024 at 04:53:16PM +0200, Alice Ryhl wrote: > On Tue, Jun 4, 2024 at 4:29 PM Will Deacon <will@xxxxxxxxxx> wrote: > > On Tue, Apr 30, 2024 at 11:09:25AM +0000, Alice Ryhl wrote: > > > Will Deacon <will@xxxxxxxxxx> wrote: > > > > On Tue, Mar 05, 2024 at 01:14:19PM +0100, Miguel Ojeda wrote: > > > >> Otherwise partially reverting to the `target.json` approach sounds good too. > > > >> > > > >> I added the `-Zuse-sync-unwind=n` to the list at > > > >> https://github.com/Rust-for-Linux/linux/issues/2. Given the default is > > > >> what we want, I have put it in the "Good to have" section. > > > > > > > > I think we have time to do this properly, like we did for the clang > > > > enablement a few years ago. In hindsight, avoiding hacks for the early > > > > toolchains back then was a really good idea because it meant we could > > > > rely on a solid baseline set of compiler features from the start. > > > > > > > > So, please can we fix this in rustc and just have SCS dependent on that? > > > > > > Just to keep you in the loop, I've posted a PR to make rustc recognize > > > the reserve-x18 target feature, so that the -Ctarget-feature=+reserve-x18 > > > flag stops emitting a warning. > > > > > > This should be sufficient for adding support for CONFIG_DYNAMIC_SCS. > > > > > > You can find it here: > > > https://github.com/rust-lang/rust/pull/124323 > > > > > > As for non-dynamic SCS, I plan to tackle that after the PR is merged. > > > See the "Future possibilities" section in the linked PR for more info on > > > that. > > > > Thanks for persevering with this, Alice. I read the pull request above, > > but it looks like you went with: > > > > https://github.com/rust-lang/rust/pull/124655 > > > > instead, which was merged (hurrah!). Do we need anything else? > > Yeah, it took a while, but I've managed to get a -Zfixed-x18 flag in. > It will be available starting with Rust 1.80, which will be released > on the 25th of July. Great, thank you! > A few things: > > 1. The -Zsanitizer=shadow-call-stack flag still doesn't work because > the compiler thinks that the target doesn't support it. I'll fix this > eventually, but at least CONFIG_DYNAMIC_SCS works now. > > 2. I haven't convinced the Rust maintainers that -Zfixed-x18 is the > way to go long term (flags starting with -Z are unstable and may > change). Some of the maintainers want to instead add a x18-available > target feature (that is, the inverse of the current reserve-x18 target > feature), that you can disable with -Ctarget-feature=-x18-available. > > And a few questions for you: > > By the time support for 1.80 goes in, we are probably supporting more > than one Rust compiler. For pre-1.80 compilers, should we fall back to > -Ctarget-feature=+reserve-x18 (which emits a warning, but works), or > fail compilation? I think we should just prevent the Kconfig option from being enabled if the toolchain doesn't give us what we need. So there's no need to fail compilation, but SCS =n. See how e.g. CONFIG_ARM64_BTI_KERNEL depends on CONFIG_CC_HAS_BRANCH_PROT_PAC_RET_BTI. > Similarly, we should probably submit a fix to the stable branches so > that SCS+Rust doesn't silently break in a hard-to-debug way. Do you > prefer a backport with -Ctarget-feature=+reserve-x18 or one that fails > compilation? As above, I think we should disable the feature in those cases. Make sense? Will
