[PATCH 1/2] riscv: Correct the initialized flow of FP register

[Vincent Chen]

  The following two reasons cause FP registers are sometimes not
initialized before starting the user program.
1. Currently, the FP context is initialized in flush_thread() function
   and we expect these initial values to be restored to FP register when
   doing FP context switch. However, the FP context switch only occurs in
   switch_to function. Hence, if this process does not be scheduled out
   and scheduled in before entering the user space, the FP registers
   have no chance to initialize.
2. In flush_thread(), the state of reg->sstatus.FS inherits from the
   parent. Hence, the state of reg->sstatus.FS may be dirty. If this
   process is scheduled out during flush_thread() and initializing the
   FP register, the fstate_save() in switch_to will corrupt the FP context
   which has been initialized until flush_thread().

  To solve the 1st case, the initialization of the FP register will be
completed in start_thread(). It makes sure all FP registers are initialized
before starting the user program. For the 2nd case, the state of
reg->sstatus.FS in start_thread will be set to SR_FS_OFF to prevent this
process from corrupting FP context in doing context save. The FP state is
set to SR_FS_INITIAL in start_trhead().

Tested on both QEMU and HiFive Unleashed using BBL + Linux.

Signed-off-by: Vincent Chen <vincent.chen@xxxxxxxxxx>
---
 arch/riscv/include/asm/switch_to.h |  6 ++++++
 arch/riscv/kernel/process.c        | 13 +++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 853b65e..d5fe573 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -19,6 +19,12 @@ static inline void __fstate_clean(struct pt_regs *regs)
 	regs->sstatus |= (regs->sstatus & ~(SR_FS)) | SR_FS_CLEAN;
 }
 
+static inline void fstate_off(struct task_struct *task,
+			       struct pt_regs *regs)
+{
+	regs->sstatus = (regs->sstatus & ~(SR_FS)) | SR_FS_OFF;
+}
+
 static inline void fstate_save(struct task_struct *task,
 			       struct pt_regs *regs)
 {
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index f23794b..e3077ee 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -64,8 +64,16 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
 	unsigned long sp)
 {
 	regs->sstatus = SR_SPIE;
-	if (has_fpu)
+	if (has_fpu) {
 		regs->sstatus |= SR_FS_INITIAL;
+#ifdef CONFIG_FPU
+		/*
+		 * Restore the initial value to the FP register
+		 * before starting the user program.
+		 */
+		fstate_restore(current, regs);
+#endif
+	}
 	regs->sepc = pc;
 	regs->sp = sp;
 	set_fs(USER_DS);
@@ -75,10 +83,11 @@ void flush_thread(void)
 {
 #ifdef CONFIG_FPU
 	/*
-	 * Reset FPU context
+	 * Reset FPU state and context
 	 *	frm: round to nearest, ties to even (IEEE default)
 	 *	fflags: accrued exceptions cleared
 	 */
+	fstate_off(current, task_pt_regs(current));
 	memset(&current->thread.fstate, 0, sizeof(current->thread.fstate));
 #endif
 }
-- 
2.7.4