fix: do not panic if virtio device activation return Err(...)

When the guest driver sets a virtio devices status to `DRIVER_OK`, we
proceed with calling `VirtioDevice::activate`. However, our MMIO
transport layer assumes that this activation cannot go wrong, and calls
`.expect(...)` on the result. For most devices, this is fine, as the
activate method doesn't do much besides writing to an event_fd (and I
can't think of a scenario in which this could go wrong). However, our
vhost-user-blk device has some non-trivial logic inside of its
`activate` method, which includes communication with the
vhost-user-backend via a unix socket. If this unix socket gets closed
early, this causes `activate` to return an error, and thus consequently
a panic in the MMIO code.

The virtio spec, in Section 2.2, has the following to say [1]:

> The device SHOULD set DEVICE_NEEDS_RESET when it enters an error state
  that a reset is needed. If DRIVER_OK is set, after it sets
  DEVICE_NEEDS_RESET, the device MUST send a device configuration
  change notification to the driver.

So the spec-conform way of handling an activation error is setting
the `DEVICE_NEEDS_RESET` flag in the device_status field (which is what
this commit does).

This will fix the panic, however it will most certainly still not result
in correct device operations (e.g. a vhost-user-backend dying will still
be unrecoverable). This is because Firecracker does not actually
implement device reset, see also firecracker-microvm#3074. Thus, the device will simply be
"dead" to the guest: All operations where we currently simply abort and
do nothing if the device is in the FAILED state will do the same in the
DEVICE_NEEDS_RESET state.

[1]: https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.pdf

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

src/vmm/src/devices/virtio/mmio.rs

@@ -11,7 +11,7 @@ use std::sync::{Arc, Mutex, MutexGuard};
 
 use utils::byte_order;
 
-use crate::devices::virtio::device::VirtioDevice;
+use crate::devices::virtio::device::{IrqType, VirtioDevice};
 use crate::devices::virtio::device_status;
 use crate::devices::virtio::queue::Queue;
 use crate::logger::warn;
@@ -186,10 +186,18 @@ impl MmioTransport {
             DRIVER_OK if self.device_status == (ACKNOWLEDGE | DRIVER | FEATURES_OK) => {
                 self.device_status = status;
                 let device_activated = self.locked_device().is_activated();
-                if !device_activated && self.are_queues_valid() {
-                    self.locked_device()
-                        .activate(self.mem.clone())
-                        .expect("Failed to activate device");
+                if !device_activated
+                    && self.are_queues_valid()
+                    && self.locked_device().activate(self.mem.clone()).is_err()
+                {
+                    self.device_status |= DEVICE_NEEDS_RESET;
+
+                    // Section 2.1.2 of the specification states that we need to send a device
+                    // configuration change interrupt
+                    let _ = self
+                        .locked_device()
+                        .interrupt_trigger()
+                        .trigger_irq(IrqType::Config);
                 }
             }
             _ if (status & FAILED) != 0 => {
@@ -306,7 +314,9 @@ impl MmioTransport {
                     0x20 => {
                         if self.check_device_status(
                             device_status::DRIVER,
-                            device_status::FEATURES_OK | device_status::FAILED,
+                            device_status::FEATURES_OK
+                                | device_status::FAILED
+                                | device_status::DEVICE_NEEDS_RESET,
                         ) {
                             self.locked_device()
                                 .ack_features_by_page(self.acked_features_select, v);
@@ -339,7 +349,10 @@ impl MmioTransport {
                 }
             }
             0x100..=0xfff => {
-                if self.check_device_status(device_status::DRIVER, device_status::FAILED) {
+                if self.check_device_status(
+                    device_status::DRIVER,
+                    device_status::FAILED | device_status::DEVICE_NEEDS_RESET,
+                ) {
                     self.locked_device().write_config(offset - 0x100, data)
                 } else {
                     warn!("can not write to device config data area before driver is ready");

src/vmm/src/devices/virtio/mod.rs

@@ -40,6 +40,7 @@ mod device_status {
     pub const FAILED: u32 = 128;
     pub const FEATURES_OK: u32 = 8;
     pub const DRIVER_OK: u32 = 4;
+    pub const DEVICE_NEEDS_RESET: u32 = 64;
 }
 
 /// Types taken from linux/virtio_ids.h.