rnull: add soft-irq completion support

rnull currently only supports direct completion. Add option for completing
requests across CPU nodes via soft IRQ or IPI.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-17-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnull/configfs.rs

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 use super::{NullBlkDevice, THIS_MODULE};
-use core::fmt::Write;
+use core::fmt::{Display, Write};
 use kernel::{
     block::mq::gen_disk::{GenDisk, GenDiskBuilder},
     c_str,
@@ -36,7 +36,7 @@ impl AttributeOperations<0> for Config {
 
     fn show(_this: &Config, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
         let mut writer = kernel::str::Formatter::new(page);
-        writer.write_str("blocksize,size,rotational\n")?;
+        writer.write_str("blocksize,size,rotational,irqmode\n")?;
         Ok(writer.bytes_written())
     }
 }
@@ -58,6 +58,7 @@ fn make_group(
                 blocksize: 1,
                 rotational: 2,
                 size: 3,
+                irqmode: 4,
             ],
         };
 
@@ -72,6 +73,7 @@ fn make_group(
                     rotational: false,
                     disk: None,
                     capacity_mib: 4096,
+                    irq_mode: IRQMode::None,
                     name: name.try_into()?,
                 }),
             }),
@@ -79,6 +81,34 @@ fn make_group(
     }
 }
 
+#[derive(Debug, Clone, Copy)]
+pub(crate) enum IRQMode {
+    None,
+    Soft,
+}
+
+impl TryFrom<u8> for IRQMode {
+    type Error = kernel::error::Error;
+
+    fn try_from(value: u8) -> Result<Self> {
+        match value {
+            0 => Ok(Self::None),
+            1 => Ok(Self::Soft),
+            _ => Err(EINVAL),
+        }
+    }
+}
+
+impl Display for IRQMode {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::None => f.write_str("0")?,
+            Self::Soft => f.write_str("1")?,
+        }
+        Ok(())
+    }
+}
+
 #[pin_data]
 pub(crate) struct DeviceConfig {
     #[pin]
@@ -92,6 +122,7 @@ struct DeviceConfigInner {
     block_size: u32,
     rotational: bool,
     capacity_mib: u64,
+    irq_mode: IRQMode,
     disk: Option<GenDisk<NullBlkDevice>>,
 }
 
@@ -121,6 +152,7 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
                 guard.block_size,
                 guard.rotational,
                 guard.capacity_mib,
+                guard.irq_mode,
             )?);
             guard.powered = true;
         } else if guard.powered && !power_op {
@@ -205,3 +237,26 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
         Ok(())
     }
 }
+
+#[vtable]
+impl configfs::AttributeOperations<4> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+        writer.write_fmt(fmt!("{}\n", this.data.lock().irq_mode))?;
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        if this.data.lock().powered {
+            return Err(EBUSY);
+        }
+
+        let text = core::str::from_utf8(page)?.trim();
+        let value = text.parse::<u8>().map_err(|_| EINVAL)?;
+
+        this.data.lock().irq_mode = IRQMode::try_from(value)?;
+        Ok(())
+    }
+}

drivers/block/rnull/rnull.rs

@@ -4,6 +4,7 @@
 
 mod configfs;
 
+use configfs::IRQMode;
 use kernel::{
     block::{
         self,
@@ -53,35 +54,44 @@ fn new(
         block_size: u32,
         rotational: bool,
         capacity_mib: u64,
+        irq_mode: IRQMode,
     ) -> Result<GenDisk<Self>> {
         let tagset = Arc::pin_init(TagSet::new(1, 256, 1), GFP_KERNEL)?;
 
+        let queue_data = Box::new(QueueData { irq_mode }, GFP_KERNEL)?;
+
         gen_disk::GenDiskBuilder::new()
             .capacity_sectors(capacity_mib << (20 - block::SECTOR_SHIFT))
             .logical_block_size(block_size)?
             .physical_block_size(block_size)?
             .rotational(rotational)
-            .build(fmt!("{}", name.to_str()?), tagset, ())
+            .build(fmt!("{}", name.to_str()?), tagset, queue_data)
     }
 }
 
+struct QueueData {
+    irq_mode: IRQMode,
+}
+
 #[vtable]
 impl Operations for NullBlkDevice {
-    type QueueData = ();
+    type QueueData = KBox<QueueData>;
 
     #[inline(always)]
-    fn queue_rq(_queue_data: (), rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
-        mq::Request::end_ok(rq)
-            .map_err(|_e| kernel::error::code::EIO)
-            // We take no refcounts on the request, so we expect to be able to
-            // end the request. The request reference must be unique at this
-            // point, and so `end_ok` cannot fail.
-            .expect("Fatal error - expected to be able to end request");
-
+    fn queue_rq(queue_data: &QueueData, rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
+        match queue_data.irq_mode {
+            IRQMode::None => mq::Request::end_ok(rq)
+                .map_err(|_e| kernel::error::code::EIO)
+                // We take no refcounts on the request, so we expect to be able to
+                // end the request. The request reference must be unique at this
+                // point, and so `end_ok` cannot fail.
+                .expect("Fatal error - expected to be able to end request"),
+            IRQMode::Soft => mq::Request::complete(rq),
+        }
         Ok(())
     }
 
-    fn commit_rqs(_queue_data: ()) {}
+    fn commit_rqs(_queue_data: &QueueData) {}
 
     fn complete(rq: ARef<mq::Request<Self>>) {
         mq::Request::end_ok(rq)