[Qemu-devel] [PATCH] target-arm: Fix regression that disabled VFP for ARMv5 CPUs

[Peter Maydell <peter.maydell@linaro.org>]

Commit 2c7ffc414 added support for honouring the CPACR coprocessor
access control register bits which may disable access to VFP
and Neon instructions. However it failed to account for the
fact that the CPACR is only present starting from the ARMv6
architecture version, so it accidentally disabled VFP completely
for ARMv5 CPUs like the ARM926. Linux would detect this as
"no VFP present" and probably fall back to its own emulation,
but other guest OSes might crash or misbehave.

This fixes bug LP:1359930.

Reported-by: Jakub Jermar <jakub@jermar.eu>
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target-arm/cpu.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/target-arm/cpu.h b/target-arm/cpu.h
index 8098b8d..659b104 100644
--- a/target-arm/cpu.h
+++ b/target-arm/cpu.h
@@ -1255,7 +1255,14 @@ static inline bool arm_singlestep_active(CPUARMState *env)
 static inline void cpu_get_tb_cpu_state(CPUARMState *env, target_ulong *pc,
                                         target_ulong *cs_base, int *flags)
 {
-    int fpen = extract32(env->cp15.c1_coproc, 20, 2);
+    int fpen;
+
+    if (arm_feature(env, ARM_FEATURE_V6)) {
+        fpen = extract32(env->cp15.c1_coproc, 20, 2);
+    } else {
+        /* CPACR doesn't exist before v6, so VFP is always accessible */
+        fpen = 3;
+    }
 
     if (is_a64(env)) {
         *pc = env->pc;
-- 
1.9.1