> Or it could use an RFLAGS bit or a bit in MXCSR or a new register, etc.
> Almost anything would be better than an MSR that is only accessible to privileged code.
ARM does that: their flag (DIT) is accessible by non-privileged code. If you know the architecture has that flag, either because your -march= is recent enough or because the operating system told you so through the hwcaps or the emulated id registers, you can use it freely without needing to switch to privileged mode through a syscall.
> Almost anything would be better than an MSR that is only accessible to privileged code.
ARM does that: their flag (DIT) is accessible by non-privileged code. If you know the architecture has that flag, either because your -march= is recent enough or because the operating system told you so through the hwcaps or the emulated id registers, you can use it freely without needing to switch to privileged mode through a syscall.