target/arm: Correct store of FPSCR value via FPCXT_S

In commit 64f863baeedc8659 we implemented the v8.1M FPCXT_S register,
but we got the write behaviour wrong. On read, this register reads
bits [27:0] of FPSCR plus the CONTROL.SFPA bit. On write, it doesn't
just write back those bits -- it writes a value to the whole FPSCR,
whose upper 4 bits are zeroes.

We also incorrectly implemented the write-to-FPSCR as a simple store
to vfp.xregs; this skips the "update the softfloat flags" part of
the vfp_set_fpscr helper so the value would read back correctly but
not actually take effect.

Fix both of these things by doing a complete write to the FPSCR
using the helper function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201210201433.26262-3-peter.maydell@linaro.org

1 files changed, 6 insertions, 6 deletions

diff --git a/target/arm/translate-vfp.c.inc b/target/arm/translate-vfp.c.inc
index 0db936084b..8b4cfd68ca 100644
--- a/target/arm/translate-vfp.c.inc
+++ b/target/arm/translate-vfp.c.inc
@@ -723,8 +723,11 @@ static bool gen_M_fp_sysreg_write(DisasContext *s, int regno,
     }
     case ARM_VFP_FPCXT_S:
     {
-        TCGv_i32 sfpa, control, fpscr;
-        /* Set FPSCR[27:0] and CONTROL.SFPA from value */
+        TCGv_i32 sfpa, control;
+        /*
+         * Set FPSCR and CONTROL.SFPA from value; the new FPSCR takes
+         * bits [27:0] from value and zeroes bits [31:28].
+         */
         tmp = loadfn(s, opaque);
         sfpa = tcg_temp_new_i32();
         tcg_gen_shri_i32(sfpa, tmp, 31);
@@ -732,11 +735,8 @@ static bool gen_M_fp_sysreg_write(DisasContext *s, int regno,
         tcg_gen_deposit_i32(control, control, sfpa,
                             R_V7M_CONTROL_SFPA_SHIFT, 1);
         store_cpu_field(control, v7m.control[M_REG_S]);
-        fpscr = load_cpu_field(vfp.xregs[ARM_VFP_FPSCR]);
-        tcg_gen_andi_i32(fpscr, fpscr, FPCR_NZCV_MASK);
         tcg_gen_andi_i32(tmp, tmp, ~FPCR_NZCV_MASK);
-        tcg_gen_or_i32(fpscr, fpscr, tmp);
-        store_cpu_field(fpscr, vfp.xregs[ARM_VFP_FPSCR]);
+        gen_helper_vfp_set_fpscr(cpu_env, tmp);
         tcg_temp_free_i32(tmp);
         tcg_temp_free_i32(sfpa);
         break;