riscv: add the mvien + mvienh CSR

[0xllx0]

Adds the mvien + mvienh CSR to represent the Machine Virtual Interrupt Enable registers.

Related: #1, #226

[romancardenas]

What do you think about returning results to notify users about potential issues with their interrupt enum?

[0xllx0]

What do you think about returning results to notify users about potential issues with their interrupt enum?

I think this is a good idea. Do you think we should also provide an example VirtualInterrupt enum, something like:

pub struct VirtualInterrupt {
    SupervisorSoftware = 1,
    SupervisorTimer = 5,
    SupervisorExternal = 9,
    Interrupt(RangedUsize<13, 63>),
}

/// SAFETY: `Interrupt` represents the virtual RISC-V interrupts
unsafe impl InterruptNumber for VirtualInterrupt {
    const MAX_INTERRUPT_NUMBER: usize = Mvien::MAX_INTERRUPT;

    #[inline]
    fn number(self) -> usize {
        match self {
            Self::SupervisorSoftware => 1,
            Self::SupervisorTimer => 5,
            Self::SupervisorExternal => 9,
            Self::Interrupt(int) => int.inner(),
        }
    }

    #[inline]
    fn from_number(value: usize) -> Result<Self> {
        match value {
            1 => Ok(Self::SupervisorSoftware),
            5 => Ok(Self::SupervisorTimer),
            9 => Ok(Self::SupervisorExternal),
            _ => RangedUsize::try_from(value).map(Self::Interrupt),
        }
    }
}

Maybe this is better left to higher-level libraries?

[romancardenas]

What do you think about returning results to notify users about potential issues with their interrupt enum?

I think this is a good idea. Do you think we should also provide an example VirtualInterrupt enum, something like:

We have this one: https://github.com/rust-embedded/riscv/blob/master/riscv/src/interrupt/supervisor.rs

There is another for machine mode.

[romancardenas]

Let me know what you think. If you agree with my comments, apply the change and let's merge it!

[0xllx0]

Let me know what you think. If you agree with my comments, apply the change and let's merge it!

Awesome! I applied the changes. Let me squash down all of the fixup commits.

[romancardenas]

LGTM