fam_wrappers: Rework doc for len field of XSAVE2

Previous doc of len field of kvm_xsave2 is outdate. Rework it to get it
back to work.

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

kvm-bindings/Cargo.toml

@@ -28,4 +28,5 @@ zerocopy = { version = "0.8.23", optional = true, features = ["derive"] }
 
 [dev-dependencies]
 bincode = {version = "2.0.1", features = ["serde"] }
+libc = "0.2.39"
 serde_json = "1.0.125"

kvm-bindings/src/x86_64/fam_wrappers.rs

@@ -136,7 +136,26 @@ pub struct kvm_xsave2 {
     /// `kvm_xsave` structure, e.g. the sum of header and FAM. Thus, this `len` field
     /// is equal to
     /// ```no_run
-    /// (KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) - sizeof::<kvm_xsave>()).div_ceil(sizeof::<__u32>())
+    /// # use kvm_bindings::{__u32, kvm_xsave, KVM_CAP_XSAVE2};
+    /// # use libc::{c_char, open, O_RDWR};
+    /// # use std::fs::File;
+    /// # use std::os::unix::io::FromRawFd;
+    /// # use vmm_sys_util::{ioctl::ioctl_with_val, ioctl_io_nr};
+    /// # const KVM_PATH: &str = "/dev/kvm\0";
+    /// # const KVMIO: u32 = 0x03;
+    /// # let kvm_fd = unsafe { open(KVM_PATH.as_ptr() as *const c_char, O_RDWR) };
+    /// # assert!(kvm_fd >= 0);
+    /// # ioctl_io_nr!(KVM_CHECK_EXTENSION, KVMIO, 0x03);
+    /// 
+    /// ((unsafe {
+    ///     ioctl_with_val(
+    ///         &File::from_raw_fd(kvm_fd),
+    ///         KVM_CHECK_EXTENSION(),
+    ///         KVM_CAP_XSAVE2.into(),
+    ///     )
+    /// }) as usize
+    ///     - size_of::<kvm_xsave>())
+    /// .div_ceil(size_of::<__u32>());
     /// ```
     pub len: usize,
     pub xsave: kvm_xsave,