target/arm: Disable SME if SVE is disabled

There is no architectural requirement that SME implies SVE, but
our implementation currently assumes it. (FEAT_SME_FA64 does
imply SVE.) So if you try to run a CPU with eg "-cpu max,sve=off"
you quickly run into an assert when the guest tries to write to
SMCR_EL1:

#6  0x00007ffff4b38e96 in __GI___assert_fail
    (assertion=0x5555566e69cb "sm", file=0x5555566e5b24 "../../target/arm/helper.c", line=6865, function=0x5555566e82f0 <__PRETTY_FUNCTION__.31> "sve_vqm1_for_el_sm") at ./assert/assert.c:101
#7  0x0000555555ee33aa in sve_vqm1_for_el_sm (env=0x555557d291f0, el=2, sm=false) at ../../target/arm/helper.c:6865
#8  0x0000555555ee3407 in sve_vqm1_for_el (env=0x555557d291f0, el=2) at ../../target/arm/helper.c:6871
#9  0x0000555555ee3724 in smcr_write (env=0x555557d291f0, ri=0x555557da23b0, value=2147483663) at ../../target/arm/helper.c:6995
#10 0x0000555555fd1dba in helper_set_cp_reg64 (env=0x555557d291f0, rip=0x555557da23b0, value=2147483663) at ../../target/arm/tcg/op_helper.c:839
#11 0x00007fff60056781 in code_gen_buffer ()

Avoid this unsupported and slightly odd combination by
disabling SME when SVE is not present.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2005
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20231127173318.674758-1-peter.maydell@linaro.org

target/arm/cpu.c

@@ -1743,6 +1743,16 @@ void arm_cpu_finalize_features(ARMCPU *cpu, Error **errp)
             return;
         }
 
+        /*
+         * FEAT_SME is not architecturally dependent on FEAT_SVE (unless
+         * FEAT_SME_FA64 is present). However our implementation currently
+         * assumes it, so if the user asked for sve=off then turn off SME also.
+         * (KVM doesn't currently support SME at all.)
+         */
+        if (cpu_isar_feature(aa64_sme, cpu) && !cpu_isar_feature(aa64_sve, cpu)) {
+            object_property_set_bool(OBJECT(cpu), "sme", false, &error_abort);
+        }
+
         arm_cpu_sme_finalize(cpu, &local_err);
         if (local_err != NULL) {
             error_propagate(errp, local_err);