target/arm: Explicitly select short-format FSR for M-profile

For M-profile, there is no guest-facing A-profile format FSR, but we
still use the env->exception.fsr field to pass fault information from
the point where a fault is raised to the code in
arm_v7m_cpu_do_interrupt() which interprets it and sets the M-profile
specific fault status registers.  So it doesn't matter whether we
fill in env->exception.fsr in the short format or the LPAE format, as
long as both sides agree.  As it happens arm_v7m_cpu_do_interrupt()
assumes short-form.

In compute_fsr_fsc() we weren't explicitly choosing short-form for
M-profile, but instead relied on it falling out in the wash because
arm_s1_regime_using_lpae_format() would be false.  This was broken in
commit 452c67a4 when we added v8R support, because we said "PMSAv8 is
always LPAE format" (as it is for v8R), forgetting that we were
implicitly using this code path on M-profile. At that point we would
hit a g_assert_not_reached():
 ERROR:../../target/arm/internals.h:549:arm_fi_to_lfsc: code should not be reached

#7  0x0000555555e055f7 in arm_fi_to_lfsc (fi=0x7fffecff9a90) at ../../target/arm/internals.h:549
#8  0x0000555555e05a27 in compute_fsr_fsc (env=0x555557356670, fi=0x7fffecff9a90, target_el=1, mmu_idx=1, ret_fsc=0x7fffecff9a1c)
    at ../../target/arm/tlb_helper.c:95
#9  0x0000555555e05b62 in arm_deliver_fault (cpu=0x555557354800, addr=268961344, access_type=MMU_INST_FETCH, mmu_idx=1, fi=0x7fffecff9a90)
    at ../../target/arm/tlb_helper.c:132
#10 0x0000555555e06095 in arm_cpu_tlb_fill (cs=0x555557354800, address=268961344, size=1, access_type=MMU_INST_FETCH, mmu_idx=1, probe=false, retaddr=0)
    at ../../target/arm/tlb_helper.c:260

The specific assertion changed when commit fcc7404eff added
"assert not M-profile" to arm_is_secure_below_el3(), because the
conditions being checked in compute_fsr_fsc() include
arm_el_is_aa64(), which will end up calling arm_is_secure_below_el3()
and asserting before we try to call arm_fi_to_lfsc():

#7  0x0000555555efaf43 in arm_is_secure_below_el3 (env=0x5555574665a0) at ../../target/arm/cpu.h:2396
#8  0x0000555555efb103 in arm_is_el2_enabled (env=0x5555574665a0) at ../../target/arm/cpu.h:2448
#9  0x0000555555efb204 in arm_el_is_aa64 (env=0x5555574665a0, el=1) at ../../target/arm/cpu.h:2509
#10 0x0000555555efbdfd in compute_fsr_fsc (env=0x5555574665a0, fi=0x7fffecff99e0, target_el=1, mmu_idx=1, ret_fsc=0x7fffecff996c)

Avoid the assertion and the incorrect FSR format selection by
explicitly making M-profile use the short-format in this function.

Fixes: 452c67a427 ("target/arm: Enable TTBCR_EAE for ARMv8-R AArch32")a
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1658
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230523131726.866635-1-peter.maydell@linaro.org

target/arm/tcg/tlb_helper.c

@@ -75,8 +75,17 @@ static uint32_t compute_fsr_fsc(CPUARMState *env, ARMMMUFaultInfo *fi,
     ARMMMUIdx arm_mmu_idx = core_to_arm_mmu_idx(env, mmu_idx);
     uint32_t fsr, fsc;
 
-    if (target_el == 2 || arm_el_is_aa64(env, target_el) ||
-        arm_s1_regime_using_lpae_format(env, arm_mmu_idx)) {
+    /*
+     * For M-profile there is no guest-facing FSR. We compute a
+     * short-form value for env->exception.fsr which we will then
+     * examine in arm_v7m_cpu_do_interrupt(). In theory we could
+     * use the LPAE format instead as long as both bits of code agree
+     * (and arm_fi_to_lfsc() handled the M-profile specific
+     * ARMFault_QEMU_NSCExec and ARMFault_QEMU_SFault cases).
+     */
+    if (!arm_feature(env, ARM_FEATURE_M) &&
+        (target_el == 2 || arm_el_is_aa64(env, target_el) ||
+         arm_s1_regime_using_lpae_format(env, arm_mmu_idx))) {
         /*
          * LPAE format fault status register : bottom 6 bits are
          * status code in the same form as needed for syndrome