Add `stac` and `clac` instructions?

[ChocolateLoverRaj]

I just discovered SMAP and am using now stac and clac instructions in my OS. Since this library kind of wraps many assembly instructions, should stac and clac also have Rust functions in this library?

[Freax13]

Good idea! Feel free to open a PR.

[ChocolateLoverRaj]

Also if run on a CPU which doesn't support SMAP, they will cause an invalid opcode fault. Will we be checking for support in the Rust fn?

[Freax13]

We have two options: we can make the functions unsafe like we do for flush_pcid or we can add a type that checks for support like RdRand.

[ChocolateLoverRaj]

Imo the function stac and clac should check CPUID and panic if unsupported. There could also be functions called stac_if_supported and clac_if_supported.

[Freax13]

Checking CPUID every time will be very expensive. That's why we use a type for RDRAND: You only have to do the check once to get an instance and then you can use it as often as you like.

[ChocolateLoverRaj]

I didn't know CPUID was an expensive operation. In that case I think having a struct that gets constructed after 1 check which has stac and clac methods is a good idea.

[Freax13]

Executing CPUID might take more than 100 cycles. It's typically implemented in microcode. When executed in a VM, it will cause a VMExit requiring hypervisor emulation and that'll be even slower. It's also a so called "serializing" instructions that prevents some CPU optimizations.