virtio-gpu: start working on the scheme

Signed-off-by: Anhad Singh <andypython@protonmail.com>

virtio-core/src/probe.rs

@@ -12,7 +12,7 @@ use syscall::{Io, PHYSMAP_NO_CACHE, PHYSMAP_WRITE};
 
 use crate::spec::*;
 use crate::transport::{Error, StandardTransport};
-use crate::utils::{VolatileCell, align_down};
+use crate::utils::{align_down, VolatileCell};
 
 pub struct Device<'a> {
     pub transport: Arc<StandardTransport<'a>>,
@@ -186,11 +186,7 @@ pub fn probe_device<'a>(pcid_handle: &mut PcidServerHandle) -> Result<Device<'a>
 
             let size = offset + capability.length as usize;
 
-            let addr = syscall::physmap(
-                aligned_addr,
-                size,
-                PHYSMAP_WRITE | PHYSMAP_NO_CACHE,
-            )?;
+            let addr = syscall::physmap(aligned_addr, size, PHYSMAP_WRITE | PHYSMAP_NO_CACHE)?;
 
             addr + offset
         };

virtio-core/src/transport.rs

@@ -86,6 +86,12 @@ impl<'a> Future for PendingRequest<'a> {
             .insert(self.first_descriptor, cx.waker().clone());
 
         let used_head = self.queue.used.head_index();
+
+        if used_head == self.queue.used_head.load(Ordering::SeqCst) {
+            // No new requests have been completed.
+            return Poll::Pending;
+        }
+
         let used_element = self.queue.used.get_element_at((used_head - 1) as usize);
         let written = used_element.written.get();
 
@@ -109,6 +115,8 @@ impl<'a> Future for PendingRequest<'a> {
                 .lock()
                 .unwrap()
                 .remove(&self.first_descriptor);
+
+            self.queue.used_head.store(used_head, Ordering::SeqCst);
             return Poll::Ready(written);
         } else {
             return Poll::Pending;
@@ -122,6 +130,7 @@ pub struct Queue<'a> {
     pub used: Used<'a>,
     pub descriptor: Dma<[Descriptor]>,
     pub available: Available<'a>,
+    pub used_head: AtomicU16,
 
     notification_bell: &'a mut AtomicU16,
     descriptor_stack: crossbeam_queue::SegQueue<u16>,
@@ -148,6 +157,7 @@ impl<'a> Queue<'a> {
             waker: Mutex::new(std::collections::HashMap::new()),
             queue_index,
             descriptor_stack,
+            used_head: AtomicU16::new(0),
             sref: sref.clone(),
         })
     }
@@ -190,8 +200,6 @@ impl<'a> Queue<'a> {
         self.notification_bell
             .store(self.queue_index, Ordering::SeqCst);
 
-        assert_eq!(self.used.flags(), 0);
-
         PendingRequest {
             queue: self.sref.upgrade().unwrap(),
             first_descriptor: first_descriptor as u32,
@@ -470,6 +478,7 @@ impl<'a> StandardTransport<'a> {
 
             event_queue
                 .add(irq_fd, move |_| -> Result<Option<usize>, std::io::Error> {
+                    // Wake up the tasks waiting on the queue.
                     for (_, task) in queue_copy.waker.lock().unwrap().iter() {
                         task.wake_by_ref();
                     }

virtio-core/src/utils.rs

@@ -1,7 +1,7 @@
 use core::cell::UnsafeCell;
+use core::fmt::Debug;
 use core::marker::PhantomData;
 
-#[derive(Debug)]
 #[repr(C)]
 pub struct VolatileCell<T> {
     value: UnsafeCell<T>,
@@ -28,6 +28,17 @@ impl<T: Copy> VolatileCell<T> {
     }
 }
 
+impl<T> Debug for VolatileCell<T>
+where
+    T: Debug + Copy,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("VolatileCell")
+            .field("value", &self.get())
+            .finish()
+    }
+}
+
 unsafe impl<T> Sync for VolatileCell<T> {}
 
 #[repr(C)]