Add support for KVM_GET_XSAVE2 ioctls

Since Linux 5.17, the `kvm_xsave` struct has a flexible array member
(FAM) at the end, which can be retrieved using the `KVM_GET_XSAVE2`
ioctl [1]. What makes this FAM special is that the length is not stored
in the header, but has to be retrieved via
`KVM_CHECK_CAPABILITY(KVM_CAP_XSAVE2)`, which returns the total size of
the `kvm_xsave` struct (e.g. the traditional 4096 byte header + the size
of the FAM). Thus, to support `KVM_GET_XSAVE2`, we first need to check
the capability on the VM fd, construct a FamStructWrapper of the correct
size, and then call `KVM_GET_XSAVE2`.

[1]: https://www.kernel.org/doc/html/latest/virt/kvm/api.html#kvm-get-xsave2

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 ### Added
 
+- [[#261](https://github.com/rust-vmm/kvm-ioctls/pull/261)]: Add support
+  for `KVM_CAP_XSAVE2` and the `KVM_GET_XSAVE2` ioctl.
+
 ### Changed
 
 ## v0.17.0

src/cap.rs

@@ -93,6 +93,8 @@ pub enum Cap {
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
     Xsave = KVM_CAP_XSAVE,
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    Xsave2 = KVM_CAP_XSAVE2,
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
     Xcrs = KVM_CAP_XCRS,
     PpcGetPvinfo = KVM_CAP_PPC_GET_PVINFO,
     PpcIrqLevel = KVM_CAP_PPC_IRQ_LEVEL,

src/ioctls/vcpu.rs

@@ -853,6 +853,41 @@ impl VcpuFd {
         Ok(xsave)
     }
 
+    /// X86 specific call that returns the vcpu's current "xsave struct" via `KVM_GET_XSAVE2`.
+    ///
+    /// See the documentation for `KVM_GET_XSAVE2` in the
+    /// [KVM API doc](https://www.kernel.org/doc/Documentation/virtual/kvm/api.txt).
+    ///
+    /// # Arguments
+    ///
+    /// * `vm_fd` - the file descriptor of the VM this [`Vcpu`] belongs to.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # extern crate kvm_ioctls;
+    /// # use kvm_ioctls::Kvm;
+    /// let kvm = Kvm::new().unwrap();
+    /// let vm = kvm.create_vm().unwrap();
+    /// let vcpu = vm.create_vcpu(0).unwrap();
+    /// let xsave = vcpu.get_xsave2(&vm).unwrap();
+    /// ```
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    pub fn get_xsave2(&self, vm_fd: &crate::VmFd) -> Result<Xsave> {
+        let xsave_size = vm_fd.xsave_size();
+        let fam_size = xsave_size - std::mem::size_of::<kvm_xsave>();
+        let mut xsave = Xsave::new(fam_size).map_err(|_| errno::Error::new(libc::EINVAL))?;
+
+        // SAFETY: Here we trust the kernel not to read past the end of the kvm_xsave struct.
+        let ret = unsafe {
+            ioctl_with_mut_ref(self, KVM_GET_XSAVE2(), &mut xsave.as_mut_fam_struct().xsave)
+        };
+        if ret != 0 {
+            return Err(errno::Error::last());
+        }
+        Ok(xsave)
+    }
+
     /// X86 specific call that sets the vcpu's current "xsave struct".
     ///
     /// See the documentation for `KVM_SET_XSAVE` in the
@@ -884,6 +919,13 @@ impl VcpuFd {
         Ok(())
     }
 
+    /// Convenience function for doing `KVM_SET_XSAVE` with the FAM-enabled [`Xsave`]
+    /// instead of the pre-5.17 plain [`kvm_xsave`].
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    pub fn set_xsave2(&self, xsave: &Xsave) -> Result<()> {
+        self.set_xsave(&xsave.as_fam_struct_ref().xsave)
+    }
+
     /// X86 specific call that returns the vcpu's current "xcrs".
     ///
     /// See the documentation for `KVM_GET_XCRS` in the
@@ -2200,6 +2242,12 @@ mod tests {
         vcpu.set_xsave(&xsave).unwrap();
         let other_xsave = vcpu.get_xsave().unwrap();
         assert_eq!(&xsave.region[..], &other_xsave.region[..]);
+        let xsave2 = vcpu.get_xsave2(&vm).unwrap();
+        assert_eq!(
+            &xsave.region[..],
+            &xsave2.as_fam_struct_ref().xsave.region[..]
+        );
+        vcpu.set_xsave2(&xsave2).unwrap();
     }
 
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]

src/ioctls/vm.rs

@@ -1293,6 +1293,20 @@ impl VmFd {
         self.run_size
     }
 
+    /// Get the `kvm_xsave` size
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    pub fn xsave_size(&self) -> usize {
+        match self.check_extension_int(Cap::Xsave2) {
+            // If KVM does not support KVM_CAP_XSAVE2, then kvm_xsave will not
+            // have a FAM field, meaning the size of the struct is just the 4096 byte header array.
+            // Otherwise, KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2) will always return at least 4096,
+            // and describe the size of the header plus the FAM.
+            // See https://docs.kernel.org/virt/kvm/api.html#kvm-get-xsave2
+            ..=0 => std::mem::size_of::<kvm_xsave>(),
+            size => size as usize,
+        }
+    }
+
     /// Wrapper over `KVM_CHECK_EXTENSION`.
     ///
     /// Returns 0 if the capability is not available and a positive integer otherwise.

src/kvm_ioctls.rs

@@ -214,6 +214,9 @@ ioctl_iow_nr!(KVM_SET_DEBUGREGS, KVMIO, 0xa2, kvm_debugregs);
 /* Available with KVM_CAP_XSAVE */
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 ioctl_ior_nr!(KVM_GET_XSAVE, KVMIO, 0xa4, kvm_xsave);
+/* Available with KVM_CAP_XSAVE2 */
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+ioctl_ior_nr!(KVM_GET_XSAVE2, KVMIO, 0xcf, kvm_xsave);
 /* Available with KVM_CAP_XSAVE */
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 ioctl_iow_nr!(KVM_SET_XSAVE, KVMIO, 0xa5, kvm_xsave);