Use `core::arch::x86_64::has_cpuid`

[JonathanWoollett-Light]

It would be preferable to use the more general core::arch::x86_64::has_cpuid to check for CPUID support.

Blocked on: rust-lang/rust#48556

[roypat]

Mh, the rust-lang issue you linked is closed, and it seems that there is some bug marking the cpuid function as nightly even though it should be stable rust-lang/rust#98253

[roypat]

The issue tracking this on rust's side has yet again changed: rust-lang/rust#60123

[roypat]

That being said, Wikipedia has this to say about the cpuid instruction:

It was introduced by Intel in 1993 with the launch of the Pentium and SL-enhanced 486 processors.[1]

And it also seems that either x86_64 CPUs without CPUID support don't exist, or even the linux kernel wouldn't work on them (https://www.geoffchappell.com/studies/windows/km/cpu/cpuid/index.htm), so why do we need to ever do such a check in the first place? 🤔

[workingjubilee]

You probably do not, and if you ever did want to actually know about the CPU's cpuid support, you would want to also ask questions about e.g. whether the cpuid_fault feature exists, or else it would be a dangerously incomplete answer. This is why has_cpuid is a bad function: it provides a useless answer to a useless question.

[roypat]

Hi @workingjubilee,
thanks for your input! I agree that this is not a question that makes sense for us to ask. We have a clearly defined set of supported architectures, and all of those have the cpuid instruction. But even beyond that, Firecracker, being a virtual machine manager, needs VMX on Intel, which is even newer than CPUID. So lots of things would go wrong before we even hit a CPUID instruction in the first place (not that I can even find a single call to __cpuid in our code anyway. I am assuming KVM internally uses it). I also see now a comment on rust-lang/rust#60123 from a few days ago that Rust doesn't have targets without CPUID anymore. Yet another reason why Firecracker cannot work on such platforms.

So I am going ahead with closing this issue :)