x86/fpu/xstate: Add fpstate_realloc()/free()

The fpstate embedded in struct fpu is the default state for storing the FPU
registers. It's sized so that the default supported features can be stored.
For dynamically enabled features the register buffer is too small.

The #NM handler detects first use of a feature which is disabled in the
XFD MSR. After handling permission checks it recalculates the size for
kernel space and user space state and invokes fpstate_realloc() which
tries to reallocate fpstate and install it.

Provide the allocator function which checks whether the current buffer size
is sufficient and if not allocates one. If allocation is successful the new
fpstate is initialized with the new features and sizes and the now enabled
features is removed from the task's XFD mask.

realloc_fpstate() uses vzalloc(). If use of this mechanism grows to
re-allocate buffers larger than 64KB, a more sophisticated allocation
scheme that includes purpose-built reclaim capability might be justified.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211021225527.10184-19-chang.seok.bae@intel.com

1 files changed, 7 insertions, 0 deletions

diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
index 798ae9225f0e..b7267b9e452f 100644
--- a/arch/x86/include/asm/fpu/api.h
+++ b/arch/x86/include/asm/fpu/api.h
@@ -130,6 +130,13 @@ static inline void fpstate_init_soft(struct swregs_state *soft) {}
 /* State tracking */
 DECLARE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
 
+/* Process cleanup */
+#ifdef CONFIG_X86_64
+extern void fpstate_free(struct fpu *fpu);
+#else
+static inline void fpstate_free(struct fpu *fpu) { }
+#endif
+
 /* fpstate-related functions which are exported to KVM */
 extern void fpstate_clear_xstate_component(struct fpstate *fps, unsigned int xfeature);