RustInQemu/2022

Notes about possibilities for including Rust code in the Qemu project. Started after discussion at the Rust in Qemu BoF at KVM Forum 2021.

Rust target support for Qemu host platforms

Information sources:

Notes:

Some of the N/A entries I'm uncertain about
- I think NetBSD lacks mips64 support, but I don't entirely know how to interpret the way they describe CPU architecture support, so again, I'm uncertain.
- I don't believe macOS or Windows have ARM32 support, but I'm not sure where to confirm that
- I believe macOS has dropped 32-bit x86 support, but I'm not sure if we support any old macOS versions which might support 32-bit x86
- It's not clear to me if qemu supports 32-bit RISC-V as a built platform
In the colouring, I've considered Tier3* as "better than" Tier2. This is debatable, but the rationale is that if we don't have host tools, that's not really sufficient for a supported qemu build platform.
I've not investigated support on Solaris or other Unix variants. Those aren't mentioned here, but I'm not really sure if there are some users of qemu on those platforms.

Legend
Tier1*	Rust Tier 1 with Host Tools supported target.
Tier2*	Rust Tier 2 with Host Tools supported target
Tier3*	Rust Tier 3 target, with full standard library, and host tool build support
Tier2	Rust Tier 2 target without host tools, but with full standard library.
Tier3	Rust Tier 3 target, but lacks either full standard library or host tool build support.
-	Rust does not have support for this target
N/A	OS doesn't support this architecture

	Linux	macOS	Windows	FreeBSD	NetBSD	OpenBSD
armv6	Tier2*	N/A?	N/A?	Tier3*	Tier3*	N/A
armv7	Tier2*	N/A?	N/A?	Tier3*	Tier3*	-
aarch64	Tier1*	Tier2*	Tier2*	Tier3*	Tier3*	Tier3*
MIPS (32-bit)	Tier2*	N/A	N/A	-	-	N/A
MIPS (64-bit)	Tier2*	N/A	N/A	-	N/A?	-
PPC (32-bit)	Tier2*	N/A	N/A	-	Tier2*	Tier3
PPC (64-bit)	Tier2*	N/A	N/A	Tier3*	N/A?	N/A
RISC-V (64-bit)	Tier2*	N/A	N/A	-	N/A	N/A
s390x	Tier2*	N/A	N/A	N/A	N/A	N/A
SPARC (32-bit)	Tier3	N/A	N/A	N/A	-	N/A
SPARC (64-bit)	Tier2	N/A	N/A	N/A	Tier3*	Tier3
x86 (32-bit)	Tier1*	N/A?	Tier1*	Tier2	Tier3*	Tier3*
x86 (64-bit)	Tier1*	Tier1*	Tier1*	Tier2*	Tier2*	Tier3*

Notes:

Qemu build platforms / distros
Package availability information from Repology
Not sure what, if anything, should count as "packaged" for Windows
Not sure how to test how easy/effective it is to use Rustup to get a toolchain on various platforms
Suitable version is currently considered Rust 1.31.0 or later
- This version is chosen since it's the first with Rust 2018 edition support, which we almost certainly want
- There may well be important features since then that we want, but I don't yet know what they are

Legend
Packaged	OS/distro has a prebuilt packages including rustc and cargo of suitable version
Rustup	Rustup works well to install toolchain on this platform
Unknown	Rust toolchain availability unknown