KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 261 insertions, 0 deletions

diff --git a/drivers/block/rnull/configfs.rs b/drivers/block/rnull/configfs.rs
new file mode 100644
index 000000000..32c10c3f4
--- /dev/null
+++ b/drivers/block/rnull/configfs.rs
@@ -0,0 +1,261 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::NullBlkDevice;
+use kernel::{
+    block::mq::gen_disk::{GenDisk, GenDiskBuilder},
+    configfs::{self, AttributeOperations},
+    configfs_attrs,
+    fmt::{self, Write as _},
+    new_mutex,
+    page::PAGE_SIZE,
+    prelude::*,
+    str::{kstrtobool_bytes, CString},
+    sync::Mutex,
+};
+
+pub(crate) fn subsystem() -> impl PinInit<kernel::configfs::Subsystem<Config>, Error> {
+    let item_type = configfs_attrs! {
+        container: configfs::Subsystem<Config>,
+        data: Config,
+        child: DeviceConfig,
+        attributes: [
+            features: 0,
+        ],
+    };
+
+    kernel::configfs::Subsystem::new(c"rnull", item_type, try_pin_init!(Config {}))
+}
+
+#[pin_data]
+pub(crate) struct Config {}
+
+#[vtable]
+impl AttributeOperations<0> for Config {
+    type Data = 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,irqmode\n")?;
+        Ok(writer.bytes_written())
+    }
+}
+
+#[vtable]
+impl configfs::GroupOperations for Config {
+    type Child = DeviceConfig;
+
+    fn make_group(
+        &self,
+        name: &CStr,
+    ) -> Result<impl PinInit<configfs::Group<DeviceConfig>, Error>> {
+        let item_type = configfs_attrs! {
+            container: configfs::Group<DeviceConfig>,
+            data: DeviceConfig,
+            attributes: [
+                // Named for compatibility with C null_blk
+                power: 0,
+                blocksize: 1,
+                rotational: 2,
+                size: 3,
+                irqmode: 4,
+            ],
+        };
+
+        Ok(configfs::Group::new(
+            name.try_into()?,
+            item_type,
+            // TODO: cannot coerce new_mutex!() to impl PinInit<_, Error>, so put mutex inside
+            try_pin_init!( DeviceConfig {
+                data <- new_mutex!(DeviceConfigInner {
+                    powered: false,
+                    block_size: 4096,
+                    rotational: false,
+                    disk: None,
+                    capacity_mib: 4096,
+                    irq_mode: IRQMode::None,
+                    name: name.try_into()?,
+                }),
+            }),
+        ))
+    }
+}
+
+#[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 fmt::Display for IRQMode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::None => f.write_str("0")?,
+            Self::Soft => f.write_str("1")?,
+        }
+        Ok(())
+    }
+}
+
+#[pin_data]
+pub(crate) struct DeviceConfig {
+    #[pin]
+    data: Mutex<DeviceConfigInner>,
+}
+
+#[pin_data]
+struct DeviceConfigInner {
+    powered: bool,
+    name: CString,
+    block_size: u32,
+    rotational: bool,
+    capacity_mib: u64,
+    irq_mode: IRQMode,
+    disk: Option<GenDisk<NullBlkDevice>>,
+}
+
+#[vtable]
+impl configfs::AttributeOperations<0> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+
+        if this.data.lock().powered {
+            writer.write_str("1\n")?;
+        } else {
+            writer.write_str("0\n")?;
+        }
+
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        let power_op = kstrtobool_bytes(page)?;
+        let mut guard = this.data.lock();
+
+        if !guard.powered && power_op {
+            guard.disk = Some(NullBlkDevice::new(
+                &guard.name,
+                guard.block_size,
+                guard.rotational,
+                guard.capacity_mib,
+                guard.irq_mode,
+            )?);
+            guard.powered = true;
+        } else if guard.powered && !power_op {
+            drop(guard.disk.take());
+            guard.powered = false;
+        }
+
+        Ok(())
+    }
+}
+
+#[vtable]
+impl configfs::AttributeOperations<1> 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().block_size))?;
+        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::<u32>().map_err(|_| EINVAL)?;
+
+        GenDiskBuilder::validate_block_size(value)?;
+        this.data.lock().block_size = value;
+        Ok(())
+    }
+}
+
+#[vtable]
+impl configfs::AttributeOperations<2> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+
+        if this.data.lock().rotational {
+            writer.write_str("1\n")?;
+        } else {
+            writer.write_str("0\n")?;
+        }
+
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        if this.data.lock().powered {
+            return Err(EBUSY);
+        }
+
+        this.data.lock().rotational = kstrtobool_bytes(page)?;
+
+        Ok(())
+    }
+}
+
+#[vtable]
+impl configfs::AttributeOperations<3> 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().capacity_mib))?;
+        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::<u64>().map_err(|_| EINVAL)?;
+
+        this.data.lock().capacity_mib = value;
+        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(())
+    }
+}