Re: [PATCH 15/22] x86/entry: Add TIF_NEED_FPU_LOAD

[Sebastian Andrzej Siewior]

On 2019-01-30 12:55:07 [+0100], Borislav Petkov wrote:
> This definitely needs to be written somewhere in
> 
> arch/x86/include/asm/fpu/internal.h
> 
> or where we decide to put the FPU handling rules.

Added:

Index: staging/arch/x86/include/asm/fpu/internal.h
===================================================================
--- staging.orig/arch/x86/include/asm/fpu/internal.h
+++ staging/arch/x86/include/asm/fpu/internal.h
@@ -537,6 +537,12 @@ static inline void __fpregs_load_activat
  *
  * The FPU context is only stored/restore for user task and ->mm is used to
  * distinguish between kernel and user threads.
+ *
+ * If TIF_NEED_FPU_LOAD is cleared then CPU's FPU registers are holding the
+ * current content of current()'s FPU register state.
+ * If TIF_NEED_FPU_LOAD is set then CPU's FPU registers may not hold current()'s
+ * FPU registers. It is required to load the register before returning to
+ * userland or using the content otherwise.
  */
 static inline void
 switch_fpu_prepare(struct fpu *old_fpu, int cpu)

Sebastian