x86/fpu: Prepare for sanitizing KVM FPU code

For the upcoming AMX support it's necessary to do a proper integration with
KVM. To avoid more nasty hackery in KVM which violate encapsulation extend
struct fpu and fpstate so the fpstate switching can be consolidated and
simplified.

Currently KVM allocates two FPU structs which are used for saving the user
state of the vCPU thread and restoring the guest state when entering
vcpu_run() and doing the reverse operation before leaving vcpu_run().

With the new fpstate mechanism this can be reduced to one extra buffer by
swapping the fpstate pointer in current::thread::fpu. This makes the
upcoming support for AMX and XFD simpler because then fpstate information
(features, sizes, xfd) are always consistent and it does not require any
nasty workarounds.

Add fpu::__task_fpstate to save the regular fpstate pointer while the task
is inside vcpu_run(). Add some state fields to fpstate to indicate the
nature of the state.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211022185312.896403942@linutronix.de

arch/x86/include/asm/fpu/types.h

@@ -322,6 +322,30 @@ struct fpstate {
 	/* @user_xfeatures:	xfeatures valid in UABI buffers */
 	u64			user_xfeatures;
 
+	/* @is_valloc:		Indicator for dynamically allocated state */
+	unsigned int		is_valloc	: 1;
+
+	/* @is_guest:		Indicator for guest state (KVM) */
+	unsigned int		is_guest	: 1;
+
+	/*
+	 * @is_confidential:	Indicator for KVM confidential mode.
+	 *			The FPU registers are restored by the
+	 *			vmentry firmware from encrypted guest
+	 *			memory. On vmexit the FPU registers are
+	 *			saved by firmware to encrypted guest memory
+	 *			and the registers are scrubbed before
+	 *			returning to the host. So there is no
+	 *			content which is worth saving and restoring.
+	 *			The fpstate has to be there so that
+	 *			preemption and softirq FPU usage works
+	 *			without special casing.
+	 */
+	unsigned int		is_confidential	: 1;
+
+	/* @in_use:		State is in use */
+	unsigned int		in_use		: 1;
+
 	/* @regs: The register state union for all supported formats */
 	union fpregs_state	regs;
 
@@ -363,6 +387,14 @@ struct fpu {
 	 */
 	struct fpstate			*fpstate;
 
+	/*
+	 * @__task_fpstate:
+	 *
+	 * Pointer to an inactive struct fpstate. Initialized to NULL. Is
+	 * used only for KVM support to swap out the regular task fpstate.
+	 */
+	struct fpstate			*__task_fpstate;
+
 	/*
 	 * @__fpstate:
 	 *
@@ -378,6 +410,16 @@ struct fpu {
 	 */
 };
 
+/*
+ * Guest pseudo FPU container
+ */
+struct fpu_guest {
+	/*
+	 * @fpstate:			Pointer to the allocated guest fpstate
+	 */
+	struct fpstate			*fpstate;
+};
+
 /*
  * FPU state configuration data. Initialized at boot time. Read only after init.
  */