virtio/
virtqueue.rs

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use alloc::collections::VecDeque;
use core::{
    mem,
    mem::MaybeUninit,
    ptr,
    ptr::{NonNull, Pointee},
};

/// A virtqueue is the mechanism used for bulk data transport to and from Virtio devices. We use the split
/// virtqueue representation - the first format of virtqueue supported by Virtio.
///
/// Buffers can be added to the virtqueue to make requests of the device. Devices then execute these requests, and
/// when complete, mark the buffers as 'used' by the device.
///
/// A split virtqueue is comprised of three "areas", each of which can be separately allocated:
///    - The Descriptor Table (of size `16 * Queue Size`)
///    - The Available Ring (of size `6 + 2 * Queue Size`)
///    - The Used Ring (of size `6 + 8 * Queue Size`)
/// The queue size is found in a transport-specific way (and is a maximum of `32768`).
pub struct Virtqueue {
    size: u16,
    free_entries: VecDeque<u16>,
    pub descriptor_table: Mapped<[Descriptor]>,
    pub available_ring: Mapped<AvailableRing>,
    pub used_ring: Mapped<UsedRing>,
}

impl Virtqueue {
    pub fn new<M>(queue_size: u16, mapper: &M) -> Virtqueue
    where
        M: Mapper,
    {
        let free_entries = (0..queue_size).collect();
        let descriptor_table = unsafe { Mapped::new_slice(queue_size as usize, mapper).assume_init() };
        let available_ring = unsafe { Mapped::new(queue_size as usize, mapper) };
        let used_ring = unsafe { Mapped::new(queue_size as usize, mapper) };

        Virtqueue { size: queue_size, free_entries, descriptor_table, available_ring, used_ring }
    }

    /// Push a descriptor into the descriptor table, returning its index. Returns `None` if there is no space left
    /// in the table.
    pub fn push_descriptor(&mut self, index: u16, descriptor: Descriptor) {
        let (_, virt) = self.descriptor_table.get(index as usize).unwrap();
        unsafe {
            ptr::write_volatile(virt.as_ptr(), descriptor);
        }
    }

    /// Make the descriptor chain starting at `index` available to the device, allowing it to start servicing the
    /// described request.
    pub fn make_descriptor_available(&mut self, index: u16) {
        let ring_index_ptr = unsafe {
            let base = self.available_ring.mapped.as_ptr() as *const u16;
            base.byte_add(mem::offset_of!(AvailableRing, index))
        };

        /*
         * XXX: this is a little confusing, and feels underspecified by the spec to me. We keep the
         * ring index running continuously, and only take the modulo with respect to the ring size
         * to work out which entry to write into.
         * TODO: what happens when the `u16` wraps around?
         */
        let ring_index = unsafe { ptr::read_volatile(ring_index_ptr) };

        // Write into the correct entry of the ring
        unsafe {
            // XXX: we can't use `offset_of` on `ring` bc its dyn-sized.
            let ring = self.available_ring.mapped.as_ptr().byte_add(4) as *mut u16;
            let address = ring.add((ring_index % self.size) as usize);
            ptr::write_volatile(address, index);
        }

        // Do a fence to make sure the device sees the update to the ring before we increment the index
        // TODO: make portable
        #[cfg(target_arch = "riscv64")]
        unsafe {
            core::arch::asm!("fence ow, ow");
        }

        // Update the ring's index
        unsafe {
            ptr::write_volatile(ring_index_ptr as *mut u16, ring_index.wrapping_add(1));
        }
    }

    pub fn alloc_descriptor(&mut self) -> Option<u16> {
        self.free_entries.pop_back()
    }

    pub fn free_descriptor(&mut self, index: u16) {
        self.free_entries.push_back(index);
    }
}

#[derive(Clone, Copy, Debug)]
#[repr(C)]
pub struct Descriptor {
    /// The guest-physical address of the buffer
    pub address: u64,
    pub len: u32,
    pub flags: DescriptorFlags,
    pub next: u16,
}

bitflags::bitflags! {
    #[derive(Clone, Copy, PartialEq, Eq, Debug)]
    #[repr(transparent)]
    pub struct DescriptorFlags: u16 {
        /// Marks a buffer as continuing in the next chained descriptor.
        const NEXT = 0b1;
        /// Marks a buffer as device write-only (if not set, the buffer is device read-only).
        const WRITE = 0b10;
        /// Marks a buffer as containing a list of buffer descriptors.
        const INDIRECT = 0b100;
    }
}

#[repr(C)]
pub struct AvailableRing {
    pub flags: u16,
    /// Where to put the next descriptor entry in the ring (modulo the queue size).
    pub index: u16,
    pub ring: [u16],
}

#[repr(C)]
pub struct UsedRing {
    pub flags: u16,
    pub index: u16,
    pub ring: [UsedRingElement],
}

#[repr(C)]
pub struct UsedRingElement {
    /// The index of the first element of the used descriptor chain.
    pub start: u32,
    /// The number of bytes written into the device-writable portion of the buffer described by the descriptor
    /// chain.
    pub length: u32,
}

/// Represents an area of physical memory that has been mapped into the virtual address space (if relevant).
// TODO: could this be some kind of common abstraction?? I feel we're really kneecapped by needing to use the same
// code from the bootloader and userspace (in the future)...
pub struct Mapped<T>
where
    T: ?Sized,
{
    pub physical: usize,
    pub mapped: NonNull<T>,
}

impl<T> Mapped<T>
where
    T: ?Sized,
{
    pub unsafe fn new<M: Mapper>(metadata: <T as Pointee>::Metadata, mapper: &M) -> Mapped<T> {
        let size = unsafe { mem::size_of_val_raw::<T>(ptr::from_raw_parts(ptr::null() as *const (), metadata)) };
        let (physical, virt) = mapper.alloc(size);

        Mapped { physical, mapped: NonNull::from_raw_parts(NonNull::new(virt as *mut ()).unwrap(), metadata) }
    }
}

impl<T> Mapped<[MaybeUninit<T>]> {
    pub fn new_slice<M: Mapper>(num_elements: usize, mapper: &M) -> Mapped<[MaybeUninit<T>]> {
        let (physical, virt) = mapper.alloc(mem::size_of::<T>() * num_elements);

        Mapped {
            physical,
            mapped: NonNull::slice_from_raw_parts(
                NonNull::new(virt as *mut MaybeUninit<T>).unwrap(),
                num_elements,
            ),
        }
    }

    pub unsafe fn assume_init(self) -> Mapped<[T]> {
        let Mapped { physical, mapped } = self;
        core::mem::forget(self);

        Mapped { physical, mapped: NonNull::slice_from_raw_parts(mapped.cast(), mapped.len()) }
    }
}

impl<T> Mapped<[T]> {
    /// Get the physical and virtual addresses of the element at `index`.
    pub fn get(&mut self, index: usize) -> Option<(usize, NonNull<T>)> {
        if index >= self.mapped.len() {
            return None;
        }

        let physical = self.physical + index * mem::size_of::<T>();
        let virt = unsafe { NonNull::new_unchecked(self.mapped.as_ptr().get_unchecked_mut(index)) };
        Some((physical, virt))
    }
}

pub trait Mapper {
    /// Allocate `size` bytes of **zeroed** memory, and return the physical and virtual addresses.
    // TODO: a real future API should provide control over whether the memory is manually zeroed - some devices
    // don't want you to randomly write zeroes to its precious MMIO
    fn alloc(&self, size: usize) -> (usize, usize);
}