target/arm: Implement FPSCR.LTPSIZE for M-profile LOB extension

If the M-profile low-overhead-branch extension is implemented, FPSCR
bits [18:16] are a new field LTPSIZE.  If MVE is not implemented
(currently always true for us) then this field always reads as 4 and
ignores writes.

These bits used to be the vector-length field for the old
short-vector extension, so we need to take care that they are not
misinterpreted as setting vec_len. We do this with a rearrangement
of the vfp_set_fpscr() code that deals with vec_len, vec_stride
and also the QC bit; this obviates the need for the M-profile
only masking step that we used to have at the start of the function.

We provide a new field in CPUState for LTPSIZE, even though this
will always be 4, in preparation for MVE, so we don't have to
come back later and split it out of the vfp.xregs[FPSCR] value.
(This state struct field will be saved and restored as part of
the FPSCR value via the vmstate_fpscr in machine.c.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201019151301.2046-11-peter.maydell@linaro.org

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 186ee621a6541d0d882fdbef3f80505117e5b8a0..07492e9f9a44640dad640d87daed50b5463cfbde 100644 (file)
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -255,6 +255,15 @@ static void arm_cpu_reset(DeviceState *dev)
         uint8_t *rom;
         uint32_t vecbase;
 
+        if (cpu_isar_feature(aa32_lob, cpu)) {
+            /*
+             * LTPSIZE is constant 4 if MVE not implemented, and resets
+             * to an UNKNOWN value if MVE is implemented. We choose to
+             * always reset to 4.
+             */
+            env->v7m.ltpsize = 4;
+        }
+
         if (arm_feature(env, ARM_FEATURE_M_SECURITY)) {
             env->v7m.secure = true;
         } else {

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index a432f301f114362ff62fb82e6738d44859d739f8..49cd5cabcf2aa4f4aa545090d37867ef60c47b38 100644 (file)
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -549,6 +549,7 @@ typedef struct CPUARMState {
         uint32_t fpdscr[M_REG_NUM_BANKS];
         uint32_t cpacr[M_REG_NUM_BANKS];
         uint32_t nsacr;
+        int ltpsize;
     } v7m;
 
     /* Information associated with an exception about to be taken:

diff --git a/target/arm/vfp_helper.c b/target/arm/vfp_helper.c
index 364856498c4793eabeecd721f25e0f17fc7727ce..01b9d8557f75b0bc2aa1dd65c6df1aaa9410c563 100644 (file)
--- a/target/arm/vfp_helper.c
+++ b/target/arm/vfp_helper.c
@@ -174,6 +174,12 @@ uint32_t HELPER(vfp_get_fpscr)(CPUARMState *env)
             | (env->vfp.vec_len << 16)
             | (env->vfp.vec_stride << 20);
 
+    /*
+     * M-profile LTPSIZE overlaps A-profile Stride; whichever of the
+     * two is not applicable to this CPU will always be zero.
+     */
+    fpscr |= env->v7m.ltpsize << 16;
+
     fpscr |= vfp_get_fpscr_from_host(env);
 
     i = env->vfp.qc[0] | env->vfp.qc[1] | env->vfp.qc[2] | env->vfp.qc[3];