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virtio-devices: block: Fix writeback mode update flow

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Virtio v1.2 says that if CONFIG_WCE is negotiated
but FLUSH is not, the device must initialize writeback to 0.
It also says that if CONFIG_WCE was not negotiated but FLUSH
was, the driver should assume presence of a writeback cache.

Introduce a pure is_writeback_enabled helper and a
set_writeback_mode helper. This makes the two call flows
explicit:

* write_config resolves the guest requested mode against the
  negotiated features before storing it back
* activate starts from the default writeback preference and then
  resolves it against the negotiated features
* reset restores the initial writeback state

This keeps the config space value and the runtime writeback flag
in sync and makes the spec driven fallback easier to follow.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
This commit is contained in:
Anatol Belski
2026-04-01 21:23:37 +02:00
committed by Rob Bradford
parent d0b253472d
commit 5ae329305a
1 changed files with 19 additions and 17 deletions
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36
virtio-devices/src/block.rs
View File

@@ -974,24 +974,24 @@ impl Block {
}
}
fn update_writeback(&mut self) {
// Use writeback from config if VIRTIO_BLK_F_CONFIG_WCE
let writeback = if self.common.feature_acked(VIRTIO_BLK_F_CONFIG_WCE.into()) {
self.config.writeback == 1
} else {
// Else check if VIRTIO_BLK_F_FLUSH negotiated
self.common.feature_acked(VIRTIO_BLK_F_FLUSH.into())
};
/// The virtio v1.2 spec says "If VIRTIO_BLK_F_CONFIG_WCE was not
/// negotiated but VIRTIO_BLK_F_FLUSH was, the driver SHOULD assume
/// presence of a writeback cache." It also says "If
/// VIRTIO_BLK_F_CONFIG_WCE is negotiated but VIRTIO_BLK_F_FLUSH is not,
/// the device MUST initialize writeback to 0."
fn is_writeback_enabled(&self, desired: bool) -> bool {
let flush = self.common.feature_acked(VIRTIO_BLK_F_FLUSH.into());
let wce = self.common.feature_acked(VIRTIO_BLK_F_CONFIG_WCE.into());
if wce { flush && desired } else { flush }
}
fn set_writeback_mode(&mut self, enabled: bool) {
self.config.writeback = enabled as u8;
self.writeback.store(enabled, Ordering::Release);
info!(
"Changing cache mode to {}",
if writeback {
"writeback"
} else {
"writethrough"
}
if enabled { "writeback" } else { "writethrough" }
);
self.writeback.store(writeback, Ordering::Release);
}
pub fn resize(&mut self, new_size: u64) -> Result<()> {
@@ -1073,8 +1073,8 @@ impl VirtioDevice for Block {
return;
}
self.config.writeback = data[0];
self.update_writeback();
let writeback = self.is_writeback_enabled(data[0] == 1);
self.set_writeback_mode(writeback);
}
fn activate(&mut self, context: crate::device::ActivationContext) -> ActivateResult {
@@ -1097,7 +1097,8 @@ impl VirtioDevice for Block {
// Recompute the barrier size from the queues that are actually activated.
self.common.paused_sync = Some(Arc::new(Barrier::new(queues.len() + 1)));
self.update_writeback();
let writeback = self.is_writeback_enabled(self.config.writeback == 1);
self.set_writeback_mode(writeback);
let mut epoll_threads = Vec::new();
let event_idx = self.common.feature_acked(VIRTIO_RING_F_EVENT_IDX.into());
@@ -1167,6 +1168,7 @@ impl VirtioDevice for Block {
fn reset(&mut self) -> Option<Arc<dyn VirtioInterrupt>> {
let result = self.common.reset();
self.set_writeback_mode(true);
event!("virtio-device", "reset", "id", &self.id);
result
}