From 0030fbe2a8f6be0a58cb2570fa51d16b01730419 Mon Sep 17 00:00:00 2001
From: Hans Christian Schmitz <git@hcsch.eu>
Date: Wed, 16 Apr 2025 08:15:26 +0200
Subject: [PATCH] feat: add traditional mem balloon device

---
 src/balloon.rs  | 33 +++++++++++++++++++++++++++++++++
 src/features.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs      |  3 ++-
 3 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 src/balloon.rs

diff --git a/src/balloon.rs b/src/balloon.rs
new file mode 100644
index 0000000..b929687
--- /dev/null
+++ b/src/balloon.rs
@@ -0,0 +1,33 @@
+use volatile::access::{ReadOnly, ReadWrite};
+use volatile_macro::VolatileFieldAccess;
+
+pub use super::features::balloon::F;
+use crate::le32;
+
+/// Traditional Memory Balloon Device Configuration Layout
+///
+/// Use [`ConfigVolatileFieldAccess`] to work with this struct.
+#[doc(alias = "virtio_balloon_config")]
+#[cfg_attr(
+    feature = "zerocopy",
+    derive(
+        zerocopy_derive::KnownLayout,
+        zerocopy_derive::Immutable,
+        zerocopy_derive::FromBytes,
+    )
+)]
+#[derive(VolatileFieldAccess)]
+#[repr(C)]
+pub struct Config {
+    #[access(ReadOnly)]
+    num_pages: le32,
+
+    #[access(ReadWrite)]
+    actual: le32,
+
+    #[access(ReadOnly)]
+    free_page_hint_cmd_id: le32,
+
+    #[access(ReadWrite)]
+    poison_val: le32,
+}
diff --git a/src/features.rs b/src/features.rs
index 1629c10..c1610ee 100644
--- a/src/features.rs
+++ b/src/features.rs
@@ -524,6 +524,55 @@ pub mod vsock {
     impl crate::FeatureBits for F {}
 }
 
+pub mod balloon {
+    use crate::le128;
+
+    feature_bits! {
+        /// Traditional Memory Balloon Device Feature Bits
+        #[doc(alias = "VIRTIO_BALLOON_F")]
+        pub struct F: le128 {
+            /// Host has to be told before pages from the balloon are used.
+            #[doc(alias = "VIRTIO_BALLOON_F_MUST_TELL_HOST")]
+            const MUST_TELL_HOST = 1 << 0;
+
+            /// A virtqueue for reporting guest memory statistics is present.
+            #[doc(alias = "VIRTIO_BALLOON_F_STATS_VQ")]
+            const STATS_VQ = 1 << 1;
+
+            /// Deflate balloon on guest out of memory condition.
+            ///
+            /// <div class="warning">
+            ///
+            /// The specification is a bit confusing on this feature, see [oasis-tcs/virtio-spec#228](https://github.com/oasis-tcs/virtio-spec/issues/228).
+            ///
+            /// </div>
+            #[doc(alias = "VIRTIO_BALLOON_F_DEFLATE_ON_OOM")]
+            const DEFLATE_ON_OOM = 1 << 2;
+
+            /// The device has support for free page hinting.
+            /// A virtqueue for providing hints as to what memory is currently free is present.
+            /// Configuration field [`free_page_hint_cmd_id`](`crate::balloon::ConfigVolatileFieldAccess::free_page_hint_cmd_id`) is valid.
+            #[doc(alias = "VIRTIO_BALLOON_F_FREE_PAGE_HINT")]
+            const FREE_PAGE_HINT = 1 << 3;
+
+            /// A hint to the device, that the driver will immediately write
+            /// [`poison_val`] to pages after deflating them.
+            /// Configuration field [`poison_val`] is valid.
+            ///
+            /// [`poison_val`]: crate::balloon::ConfigVolatileFieldAccess::poison_val
+            #[doc(alias = "VIRTIO_BALLOON_F_PAGE_POISON")]
+            const PAGE_POISON = 1 << 4;
+
+            /// The device has support for free page reporting.
+            /// A virtqueue for reporting free guest memory is present.
+            #[doc(alias = "VIRTIO_BALLOON_F_PAGE_REPORTING")]
+            const PAGE_REPORTING = 1 << 5;
+        }
+    }
+
+    impl crate::FeatureBits for F {}
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src/lib.rs b/src/lib.rs
index 1f4b08f..b7c3708 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -67,7 +67,7 @@
 //! | Block Device                      | ❌        |             |
 //! | Console Device                    | ✅        | [`console`] |
 //! | Entropy Device                    | ❌        |             |
-//! | Traditional Memory Balloon Device | ❌        |             |
+//! | Traditional Memory Balloon Device | ✅        | [`balloon`] |
 //! | SCSI Host Device                  | ❌        |             |
 //! | GPU Device                        | ❌        |             |
 //! | Input Device                      | ❌        |             |
@@ -94,6 +94,7 @@ extern crate alloc;
 mod bitflags;
 #[macro_use]
 pub mod volatile;
+pub mod balloon;
 pub mod console;
 #[cfg(any(feature = "mmio", feature = "pci"))]
 mod driver_notifications;