x86/fpu: Provide struct fpstate

New xfeatures will not longer be automatically stored in the regular XSAVE
buffer in thread_struct::fpu.

The kernel will provide the default sized buffer for storing the regular
features up to AVX512 in thread_struct::fpu and if a task requests to use
one of the new features then the register storage has to be extended.

The state will be accessed via a pointer in thread_struct::fpu which
defaults to the builtin storage and can be switched when extended storage
is required.

To avoid conditionals all over the code, create a new container for the
register storage which will gain other information, e.g. size, feature
masks etc., later. For now it just contains the register storage, which
gives it exactly the same layout as the exiting fpu::state.

Stick fpu::state and the new fpu::__fpstate into an anonymous union and
initialize the pointer. Add build time checks to validate that both are
at the same place and have the same size.

This allows step by step conversion of all users.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211013145322.234458659@linutronix.de

1 files changed, 19 insertions, 1 deletions

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index f5a38a5f3ae1..3bb6277efbb5 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -309,6 +309,13 @@ union fpregs_state {
 	u8 __padding[PAGE_SIZE];
 };
 
+struct fpstate {
+	/* @regs: The register state union for all supported formats */
+	union fpregs_state		regs;
+
+	/* @regs is dynamically sized! Don't add anything after @regs! */
+} __aligned(64);
+
 /*
  * Highest level per task FPU state data structure that
  * contains the FPU register state plus various FPU
@@ -337,6 +344,14 @@ struct fpu {
 	unsigned long			avx512_timestamp;
 
 	/*
+	 * @fpstate:
+	 *
+	 * Pointer to the active struct fpstate. Initialized to
+	 * point at @__fpstate below.
+	 */
+	struct fpstate			*fpstate;
+
+	/*
 	 * @state:
 	 *
 	 * In-memory copy of all FPU registers that we save/restore
@@ -345,7 +360,10 @@ struct fpu {
 	 * copy. If the task context-switches away then they get
 	 * saved here and represent the FPU state.
 	 */
-	union fpregs_state		state;
+	union {
+		struct fpstate			__fpstate;
+		union fpregs_state		state;
+	};
 	/*
 	 * WARNING: 'state' is dynamically-sized.  Do not put
 	 * anything after it here.