pci: Support injecting interrupts from externally-provided irqfds

The virtio vhost-user device backend prefers to use externally-provided
eventfds as irqfds.  This allows the frontend VM to notify the backend
VM directly, without the need for a userspace proxy process.  Since the
frontend can provide irqfds at any time, the backend needs to register
and unregister irqfds dynamically.

This is tricky because the functions that access the irqfd table all
take `&self`, not `&mut self`.  The obvious solution to this problem is
to wrap the table in a mutex.  Most of these functions are not called on
hot paths, but `.notifier()` is called whenever Cloud Hypervisor needs
to inject an interrupt into a guest.  Most devices don't need to
register irqfds at runtime, and for them, slowing down interrupt
injection would be wasteful.

Instead, require devices to opt-in to irqfd registration.  The irqfd
table now comes in two forms: one that contains a mutex and one that
does not.  The one containing a mutex can be mutated freely, while
attempting to mutate the one that does not will panic.

Right now, no code registeres irqfds at runtime, but this will change in
subsequent commits.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>

vm-device/src/interrupt/mod.rs

@@ -57,7 +57,8 @@
 //! * The virtual device backend requests the interrupt manager to create an interrupt group
 //!   according to guest configuration information
 
-use std::sync::Arc;
+use std::io::{Error, ErrorKind};
+use std::sync::{Arc, Mutex};
 
 pub use hypervisor::{InterruptSourceConfig, LegacyIrqSourceConfig, MsiIrqSourceConfig};
 use vmm_sys_util::eventfd::EventFd;
@@ -107,6 +108,30 @@ pub trait InterruptManager: Send + Sync {
     /// * count: number of Interrupt Sources to be managed by the group object.
     fn create_group(&self, config: Self::GroupConfig) -> Result<Arc<dyn InterruptSourceGroup>>;
 
+    /// Create an [InterruptSourceGroup](trait.InterruptSourceGroup.html) object to manage
+    /// interrupt sources for a virtual device
+    ///
+    /// An [InterruptSourceGroup](trait.InterruptSourceGroup.html) object manages all interrupt
+    /// sources of the same type for a virtual device.
+    ///
+    /// This is the same as [`Self::create_group`], except that the returned
+    /// [`InterruptSourceGroup`] allows setting the irqfd used as notifier via
+    /// [`InterruptSourceGroup::set_notifier`].
+    ///
+    /// # Arguments
+    /// * interrupt_type: type of interrupt source.
+    /// * base: base Interrupt Source ID to be managed by the group object.
+    /// * count: number of Interrupt Sources to be managed by the group object.
+    fn create_group_mut(
+        &self,
+        _config: Self::GroupConfig,
+    ) -> Result<Arc<Mutex<dyn InterruptSourceGroup>>> {
+        Err(Error::new(
+            ErrorKind::Unsupported,
+            "setting notifiers not supported",
+        ))
+    }
+
     /// Destroy an [InterruptSourceGroup](trait.InterruptSourceGroup.html) object created by
     /// [create_group()](trait.InterruptManager.html#tymethod.create_group).
     ///
@@ -137,7 +162,7 @@ pub trait InterruptSourceGroup: Send + Sync {
     /// Returns an interrupt notifier from this interrupt.
     ///
     /// An interrupt notifier allows for external components and processes
-    /// to inject interrupts into a guest, by writing to the file returned
+    /// to inject interrupts into a guest, by writing to the [`EventFd`] returned
     /// by this method.
     #[allow(unused_variables)]
     fn notifier(&self, index: InterruptIndex) -> Option<EventFd>;
@@ -159,4 +184,17 @@ pub trait InterruptSourceGroup: Send + Sync {
 
     /// Set the interrupt group GSI routing table.
     fn set_gsi(&self) -> Result<()>;
+
+    /// Sets the [`EventFd`] used to trigger interrupts.
+    fn set_notifier(
+        &mut self,
+        _index: InterruptIndex,
+        _eventfd: Option<EventFd>,
+        _vm: &dyn hypervisor::Vm,
+    ) -> Result<()> {
+        Err(Error::new(
+            ErrorKind::Unsupported,
+            "setting notifiers not supported",
+        ))
+    }
 }