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misc: Use a more relaxed memory model when possible

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When a total ordering between multiple atomic variables is not required
then use Ordering::Acquire with atomic loads and Ordering::Release with
atomic stores.

This will improve performance as this does not require a memory fence
on x86_64 which Ordering::SeqCst will use.

Add a comment to the code in the vCPU handling code where it operates on
multiple atomics to explain why Ordering::SeqCst is required.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>
This commit is contained in:
Rob Bradford
2020-12-01 16:15:26 +00:00
committed by Samuel Ortiz
parent 280d4fb245
commit ffaab46934
11 changed files with 32 additions and 27 deletions
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virtio-devices/src/transport/pci_device.rs
+6 -6
View File
@@ -448,7 +448,7 @@ impl VirtioPciDevice {
fn state(&self) -> VirtioPciDeviceState {
VirtioPciDeviceState {
device_activated: self.device_activated,
interrupt_status: self.interrupt_status.load(Ordering::SeqCst),
interrupt_status: self.interrupt_status.load(Ordering::Acquire),
queues: self.queues.clone(),
}
}
@@ -456,7 +456,7 @@ impl VirtioPciDevice {
fn set_state(&mut self, state: &VirtioPciDeviceState) -> std::result::Result<(), Error> {
self.device_activated = state.device_activated;
self.interrupt_status
.store(state.interrupt_status, Ordering::SeqCst);
.store(state.interrupt_status, Ordering::Release);
// Update virtqueues indexes for both available and used rings.
if let Some(mem) = self.memory.as_ref() {
@@ -685,7 +685,7 @@ impl VirtioInterrupt for VirtioInterruptMsix {
queue: Option<&Queue>,
) -> std::result::Result<(), std::io::Error> {
let vector = match int_type {
VirtioInterruptType::Config => self.config_vector.load(Ordering::SeqCst),
VirtioInterruptType::Config => self.config_vector.load(Ordering::Acquire),
VirtioInterruptType::Queue => {
if let Some(q) = queue {
q.vector
@@ -717,7 +717,7 @@ impl VirtioInterrupt for VirtioInterruptMsix {
fn notifier(&self, int_type: &VirtioInterruptType, queue: Option<&Queue>) -> Option<&EventFd> {
let vector = match int_type {
VirtioInterruptType::Config => self.config_vector.load(Ordering::SeqCst),
VirtioInterruptType::Config => self.config_vector.load(Ordering::Acquire),
VirtioInterruptType::Queue => {
if let Some(q) = queue {
q.vector
@@ -891,7 +891,7 @@ impl PciDevice for VirtioPciDevice {
o if ISR_CONFIG_BAR_OFFSET <= o && o < ISR_CONFIG_BAR_OFFSET + ISR_CONFIG_SIZE => {
if let Some(v) = data.get_mut(0) {
// Reading this register resets it to 0.
*v = self.interrupt_status.swap(0, Ordering::SeqCst) as u8;
*v = self.interrupt_status.swap(0, Ordering::AcqRel) as u8;
}
}
o if DEVICE_CONFIG_BAR_OFFSET <= o
@@ -936,7 +936,7 @@ impl PciDevice for VirtioPciDevice {
o if ISR_CONFIG_BAR_OFFSET <= o && o < ISR_CONFIG_BAR_OFFSET + ISR_CONFIG_SIZE => {
if let Some(v) = data.get(0) {
self.interrupt_status
.fetch_and(!(*v as usize), Ordering::SeqCst);
.fetch_and(!(*v as usize), Ordering::AcqRel);
}
}
o if DEVICE_CONFIG_BAR_OFFSET <= o