[Qemu-devel] [PATCH 04/26] target/arm: Disable most VFP sysregs for M-profile

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

The only "system register" that M-profile floating point exposes
via the VMRS/VMRS instructions is FPSCR, and it does not have
the odd special case for rd==15. Add a check to ensure we only
expose FPSCR.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/translate.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/target/arm/translate.c b/target/arm/translate.c
index d408e4d7ef4..d56488ec847 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -3512,12 +3512,27 @@ static int disas_vfp_insn(DisasContext *s, uint32_t insn)
                     }
                 }
             } else { /* !dp */
+                bool is_sysreg;
+
                 if ((insn & 0x6f) != 0x00)
                     return 1;
                 rn = VFP_SREG_N(insn);
+
+                is_sysreg = extract32(insn, 21, 1);
+
+                if (arm_dc_feature(s, ARM_FEATURE_M)) {
+                    /*
+                     * The only M-profile VFP vmrs/vmsr sysreg is FPSCR.
+                     * Writes to R15 are UNPREDICTABLE; we choose to undef.
+                     */
+                    if (is_sysreg && (rd == 15 || (rn >> 1) != ARM_VFP_FPSCR)) {
+                        return 1;
+                    }
+                }
+
                 if (insn & ARM_CP_RW_BIT) {
                     /* vfp->arm */
-                    if (insn & (1 << 21)) {
+                    if (is_sysreg) {
                         /* system register */
                         rn >>= 1;
 
@@ -3584,7 +3599,7 @@ static int disas_vfp_insn(DisasContext *s, uint32_t insn)
                     }
                 } else {
                     /* arm->vfp */
-                    if (insn & (1 << 21)) {
+                    if (is_sysreg) {
                         rn >>= 1;
                         /* system register */
                         switch (rn) {
-- 
2.20.1