[PATCH for-8.0] target/arm: Fix generated code for cpreg reads when HSTR is active

[Peter Maydell]

In commit 049edada we added some code to handle HSTR_EL2 traps, which
we did as an inline "conditionally branch over a
gen_exception_insn()".  Unfortunately this fails to take account of
the fact that gen_exception_insn() will set s->base.is_jmp to
DISAS_NORETURN.  That means that at the end of the TB we won't
generate the necessary code to handle the "branched over the trap and
continued normal execution" codepath.  The result is that the TCG
main loop thinks that we stopped execution of the TB due to a
situation that only happens when icount is enabled, and hits an
assertion.

Note that this only happens for cpreg reads; writes will call
gen_lookup_tb() which generates a valid end-of-TB.

Fixes: 049edada ("target/arm: Make HSTR_EL2 traps take priority over 
UNDEF-at-EL1")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1551
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
saving and restoring is_jmp around the call seems super
clunky -- is there a better way ? I think mostly we avoid
this by not doing conditional exception-generation in
inline TCG code...
---
 target/arm/tcg/translate.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/target/arm/tcg/translate.c b/target/arm/tcg/translate.c
index 2cb9368b1ba..bb502165c39 100644
--- a/target/arm/tcg/translate.c
+++ b/target/arm/tcg/translate.c
@@ -4617,12 +4617,20 @@ static void do_coproc_insn(DisasContext *s, int cpnum, 
int is64,
             /* T4 and T14 are RES0 so never cause traps */
             TCGv_i32 t;
             DisasLabel over = gen_disas_label(s);
+            DisasJumpType old_is_jmp;
 
             t = load_cpu_offset(offsetoflow32(CPUARMState, cp15.hstr_el2));
             tcg_gen_andi_i32(t, t, 1u << maskbit);
             tcg_gen_brcondi_i32(TCG_COND_EQ, t, 0, over.label);
 
+            /*
+             * gen_exception_insn() will set is_jmp to DISAS_NORETURN,
+             * but since we're conditionally branching over it, we want
+             * to retain the existing value.
+             */
+            old_is_jmp = s->base.is_jmp;
             gen_exception_insn(s, 0, EXCP_UDEF, syndrome);
+            s->base.is_jmp = old_is_jmp;
             set_disas_label(s, over);
         }
     }
-- 
2.34.1