More recent kernels sometimes expose new registers in an
unconditionnal manner. This situation breaks backward migration
as qemu notices there are more registers in the input stream
than supported on the destination host. This leads to a
"failed to load cpu:cpreg_vmstate_array_len" error.

A good example is the introduction of KVM_REG_ARM_VENDOR_HYP_BMAP_2
pseudo FW register in v6.16 by commit C0000e58c74e (“KVM: arm64:
Introduce KVM_REG_ARM_VENDOR_HYP_BMAP_2”). Trying to do backward
migration from a host kernel that features the commit to a destination
host that doesn't, fail with above error.

Currently QEMU is not using that feature so ignoring this latter
is not a problem. An easy way to fix the migration issue is to teach
qemu we don't care about that register and we can simply ignore it
when syncing its state during migration.

This patch introduces an array of such hidden registers. Soon it will
be settable through an array property.

If hidden, the register is moved out of the array of cpreg which is
built in kvm_arm_init_cpreg_list(). That way their state won't be
synced.

Signed-off-by: Eric Auger <eric.auger@redhat.com>

---

v1 -> v2:
- Move the property in a separate patch
- improve the commit msg
- change the trace point to just print info in
  kvm_arm_init_cpreg_list()
- improve comment in cpu.h (Connie)
---
 target/arm/cpu.h        | 11 +++++++++++
 target/arm/kvm.c        | 31 ++++++++++++++++++++++++++++++-
 target/arm/trace-events |  2 ++
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index a7ed3f34f8..75cf7ee506 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -1041,6 +1041,17 @@ struct ArchCPU {
     /* KVM steal time */
     OnOffAuto kvm_steal_time;
 
+    /*
+     * KVM registers that must be ignored/hidden. While they may be
+     * exposed by KVM to userspace, they are not intended to be used
+     * by qemu and more importantly we don't want them to be migrated
+     * to another host which wouldn't expose them. This would break
+     * the migration. This may be useful to allow backward migration to
+     * older kernels with less features.
+     */
+    uint64_t *kvm_hidden_regs;
+    uint32_t nr_kvm_hidden_regs;
+
     /* Uniprocessor system with MP extensions */
     bool mp_is_up;
 
diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index 58c6075a9e..60fb743423 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -754,6 +754,25 @@ static bool kvm_arm_reg_syncs_via_cpreg_list(uint64_t regidx)
     }
 }
 
+/**
+ * kvm_vcpu_compat_hidden_reg:
+ * @cpu: ARMCPU
+ * @regidx: index of the register to check
+ *
+ * Depending on the CPU compat returns true if @regidx must be
+ * ignored during sync & migration
+ */
+static inline bool
+kvm_vcpu_compat_hidden_reg(ARMCPU *cpu, uint64_t regidx)
+{
+    for (int i = 0; i < cpu->nr_kvm_hidden_regs; i++) {
+        if (cpu->kvm_hidden_regs[i] == regidx) {
+            return true;
+        }
+    }
+    return false;
+}
+
 /**
  * kvm_arm_init_cpreg_list:
  * @cpu: ARMCPU
@@ -788,7 +807,10 @@ static int kvm_arm_init_cpreg_list(ARMCPU *cpu)
     qsort(&rlp->reg, rlp->n, sizeof(rlp->reg[0]), compare_u64);
 
     for (i = 0, arraylen = 0; i < rlp->n; i++) {
-        if (!kvm_arm_reg_syncs_via_cpreg_list(rlp->reg[i])) {
+        uint64_t regidx = rlp->reg[i];
+
+        if (!kvm_arm_reg_syncs_via_cpreg_list(regidx) ||
+            kvm_vcpu_compat_hidden_reg(cpu, regidx)) {
             continue;
         }
         switch (rlp->reg[i] & KVM_REG_SIZE_MASK) {
@@ -804,6 +826,8 @@ static int kvm_arm_init_cpreg_list(ARMCPU *cpu)
         arraylen++;
     }
 
+    trace_kvm_arm_init_cpreg_list_arraylen(arraylen);
+
     cpu->cpreg_indexes = g_renew(uint64_t, cpu->cpreg_indexes, arraylen);
     cpu->cpreg_values = g_renew(uint64_t, cpu->cpreg_values, arraylen);
     cpu->cpreg_vmstate_indexes = g_renew(uint64_t, cpu->cpreg_vmstate_indexes,
@@ -815,9 +839,14 @@ static int kvm_arm_init_cpreg_list(ARMCPU *cpu)
 
     for (i = 0, arraylen = 0; i < rlp->n; i++) {
         uint64_t regidx = rlp->reg[i];
+
         if (!kvm_arm_reg_syncs_via_cpreg_list(regidx)) {
             continue;
         }
+        if (kvm_vcpu_compat_hidden_reg(cpu, regidx)) {
+            trace_kvm_arm_init_cpreg_list_skip_hidden_reg(rlp->reg[i]);
+            continue;
+        }
         cpu->cpreg_indexes[arraylen] = regidx;
         arraylen++;
     }
diff --git a/target/arm/trace-events b/target/arm/trace-events
index c497df8fe7..31386cc1f2 100644
--- a/target/arm/trace-events
+++ b/target/arm/trace-events
@@ -14,6 +14,8 @@ arm_gt_update_irq(int timer, int irqstate) "gt_update_irq: timer %d irqstate %d"
 # kvm.c
 kvm_arm_fixup_msi_route(uint64_t iova, uint64_t gpa) "MSI iova = 0x%"PRIx64" is translated into 0x%"PRIx64
 kvm_arm_cpu_post_load_missing_reg(char *name) "Missing register in input stream: %s"
+kvm_arm_init_cpreg_list_arraylen(uint32_t arraylen) "arraylen=%d"
+kvm_arm_init_cpreg_list_skip_hidden_reg(uint64_t regidx) "hidden 0x%"PRIx64" is skipped"
 
 # cpu.c
 arm_cpu_reset(uint64_t mp_aff) "cpu %" PRIu64
-- 
2.49.0