Always read the VFP regs in the arm64 fill_fpregs

The PCB_FP_STARTED is used to indicate that the current VFP context
has been used since either 1. the start of the thread, or 2. exiting
a kernel FP context.

When case 2 was added to the kernel this could cause incorrect results
to be returned when a thread exits the kernel FP context and fill_fpregs
is called before it has restored the VFP state, e.g. by trappin on a
userspace VFP instruction.

In both of the cases the base save area is still valid so reduce the
use of the PCB_FP_STARTED flag check to help decide if we need to
store the current threads VFP state.

Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D37994

1 files changed, 9 insertions, 8 deletions

diff --git a/sys/arm64/arm64/exec_machdep.c b/sys/arm64/arm64/exec_machdep.c
index 6109a866a2d0..12c23149ec7f 100644
--- a/sys/arm64/arm64/exec_machdep.c
+++ b/sys/arm64/arm64/exec_machdep.c
@@ -153,16 +153,17 @@ fill_fpregs(struct thread *td, struct fpreg *regs)
 		 */
 		if (td == curthread)
 			vfp_save_state(td, pcb);
+	}
 
-		KASSERT(pcb->pcb_fpusaved == &pcb->pcb_fpustate,
-		    ("Called fill_fpregs while the kernel is using the VFP"));
-		memcpy(regs->fp_q, pcb->pcb_fpustate.vfp_regs,
-		    sizeof(regs->fp_q));
-		regs->fp_cr = pcb->pcb_fpustate.vfp_fpcr;
-		regs->fp_sr = pcb->pcb_fpustate.vfp_fpsr;
-	} else
+	KASSERT(pcb->pcb_fpusaved == &pcb->pcb_fpustate,
+	    ("Called fill_fpregs while the kernel is using the VFP"));
+	memcpy(regs->fp_q, pcb->pcb_fpustate.vfp_regs,
+	    sizeof(regs->fp_q));
+	regs->fp_cr = pcb->pcb_fpustate.vfp_fpcr;
+	regs->fp_sr = pcb->pcb_fpustate.vfp_fpsr;
+#else
+	memset(regs, 0, sizeof(*regs));
 #endif
-		memset(regs, 0, sizeof(*regs));
 	return (0);
 }