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
- 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.
Rust doesn't support 32-bit x86 on macOS, but macOS itself dropped 32-bit starting with Catalina, and QEMU doesn't support any older macOS versions than that. So it's N/A for us.

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
- When relevant updates are included, not just the distro as released
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 column)
<version>	OS/distro has a prebuilt packages including rustc and cargo, version 1.31.0 or later
<version>	OS/distro has a prebuilt package, earlier than version 1.31.0
N/A	No standard packaging for this OS

Legend (Rustup column)
Tier1	Rustup is expected to work on this platform. This means that the platform is in Rust's Tier1 with host tools list for at least one architecture, and has sufficiently recent kernel and libc versions to support it. (These criteria suggested by Peter Maydell)
Tier2	Rustup is expected to work on this platform. This means that the platform is in Rust's Tier2 with host tools list for at least one architecture, and the distro has sufficiently recent kernel and libc versions packaged. (These criteria suggested by Peter Maydell)
Tier3	Rustup might on this platform. This means that the platform is in Rust's Tier3 list with host tools list for at least one architecture, and the distro has sufficiently recent kernel and libc versions packaged.
Tier1-	Rustup might work on this platform with some work. It's on the tier 1, 2 or 3 list with host tools support, but the distro does not have the necessary kernel and/or libc packaged.
Unknown	Rust toolchain availability unknown