target/arm/helper: Define cpreg migration tolerance for DGBDTR_EL0

We want to remove AArch32 DBGDTRTX which was erroneously exposed.
This was attempted by 655659a74a36b ("target/arm: Correct encoding
of Debug Communications Channel registers") but it was discovered
that the removal of this debug register broke forward migration on
TCG. Now we have the cpreg migration tolerance infrastructure, we
can declare one for the DBGDTRTX. This allow to revert the reinstate
patch.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Sebastian Ott <sebott@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20260420140552.104369-7-eric.auger@redhat.com
[PMM: revised comment, included note about when we can drop
 the workaround]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 3ac88078aa1427ca319e888271a0af0d4e4561a2..ccd6353190552116c6abc2ba42b19eba94aa88bd 100644 (file)
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -6349,9 +6349,32 @@ void register_cp_regs_for_features(ARMCPU *cpu)
             .fgt = FGT_CLIDR_EL1,
             .resetvalue = GET_IDREG(isar, CLIDR)
         };
+        uint64_t dbgtr_el0_kvmidx =
+            cpreg_to_kvm_id(ENCODE_CP_REG(14, 0, 1, 0, 5, 3, 0));
+
         define_one_arm_cp_reg(cpu, &clidr);
         define_arm_cp_regs(cpu, v7_cp_reginfo);
         define_debug_regs(cpu);
+        /*
+         * We used to incorrectly expose a non-existent AArch32 "DBGDTRTX"
+         * register with this encoding. This has been fixed by commit
+         * 655659a74a36 ("target/arm: Correct encoding of Debug
+         * Communications Channel registers") by the introduction of correct
+         * separate cpreg definitions for AA64 and AA32 versions. However,
+         * the old cpreg definition couldn't be removed without breaking
+         * migration, so commit 4f2b82f604 reinstated the bogus encoding
+         * for migration data only.
+         *
+         * Now that we have migration tolerance infrastructure, we can use
+         * this to allow forward migration from the buggy QEMU versions,
+         * accepting and ignoring the bogus register if it is in the
+         * source data. QEMU 11.0 was the last version that sent the
+         * bogus encoding, so this workaround can be removed at the point
+         * where we no longer care about migration from that version
+         * (i.e. when we remove the "virt-11.0" machine type).
+         */
+        arm_register_cpreg_mig_tolerance(cpu, dbgtr_el0_kvmidx,
+                                         0, 0, ToleranceNotOnBothEnds);
     } else {
         define_arm_cp_regs(cpu, not_v7_cp_reginfo);
     }