[PULL 17/27] target/arm: Enforce that FPDSCR.LTPSIZE is 4 on inbound migration

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

Architecturally, for an M-profile CPU with the LOB feature the
LTPSIZE field in FPDSCR is always constant 4.  QEMU's implementation
enforces this everywhere, except that we don't check that it is true
in incoming migration data.

We're going to add come in gen_update_fp_context() which relies on
the "always 4" property.  Since this is TCG-only, we don't actually
need to be robust to bogus incoming migration data, and the effect of
it being wrong would be wrong code generation rather than a QEMU
crash; but if it did ever happen somehow it would be very difficult
to track down the cause.  Add a check so that we fail the inbound
migration if the FPDSCR.LTPSIZE value is incorrect.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210913095440.13462-3-peter.maydell@linaro.org
---
 target/arm/machine.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/target/arm/machine.c b/target/arm/machine.c
index 81e30de8243..c74d8c3f4b3 100644
--- a/target/arm/machine.c
+++ b/target/arm/machine.c
@@ -781,6 +781,19 @@ static int cpu_post_load(void *opaque, int version_id)
     hw_breakpoint_update_all(cpu);
     hw_watchpoint_update_all(cpu);
 
+    /*
+     * TCG gen_update_fp_context() relies on the invariant that
+     * FPDSCR.LTPSIZE is constant 4 for M-profile with the LOB extension;
+     * forbid bogus incoming data with some other value.
+     */
+    if (arm_feature(env, ARM_FEATURE_M) && cpu_isar_feature(aa32_lob, cpu)) {
+        if (extract32(env->v7m.fpdscr[M_REG_NS],
+                      FPCR_LTPSIZE_SHIFT, FPCR_LTPSIZE_LENGTH) != 4 ||
+            extract32(env->v7m.fpdscr[M_REG_S],
+                      FPCR_LTPSIZE_SHIFT, FPCR_LTPSIZE_LENGTH) != 4) {
+            return -1;
+        }
+    }
     if (!kvm_enabled()) {
         pmu_op_finish(&cpu->env);
     }
-- 
2.20.1