target-arm: kvm: Differentiate registers based on write-back levels

Some registers like the CNTVCT register should only be written to the
kernel as part of machine initialization or on vmload operations, but
never during runtime, as this can potentially make time go backwards or
create inconsistent time observations between VCPUs.

Introduce a list of registers that should not be written back at runtime
and check this list on syncing the register state to the KVM state.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Message-id: 1437046488-10773-1-git-send-email-christoffer.dall@linaro.org
[PMM: tweaked a few comments, added the new argument to the stub
 write_list_to_kvmstate() in target-arm/kvm-stub.c]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 29 insertions, 1 deletions

diff --git a/target-arm/kvm64.c b/target-arm/kvm64.c
index ac34f51..bd60889 100644
--- a/target-arm/kvm64.c
+++ b/target-arm/kvm64.c
@@ -139,6 +139,34 @@ bool kvm_arm_reg_syncs_via_cpreg_list(uint64_t regidx)
     }
 }
 
+typedef struct CPRegStateLevel {
+    uint64_t regidx;
+    int level;
+} CPRegStateLevel;
+
+/* All system registers not listed in the following table are assumed to be
+ * of the level KVM_PUT_RUNTIME_STATE. If a register should be written less
+ * often, you must add it to this table with a state of either
+ * KVM_PUT_RESET_STATE or KVM_PUT_FULL_STATE.
+ */
+static const CPRegStateLevel non_runtime_cpregs[] = {
+    { KVM_REG_ARM_TIMER_CNT, KVM_PUT_FULL_STATE },
+};
+
+int kvm_arm_cpreg_level(uint64_t regidx)
+{
+    int i;
+
+    for (i = 0; i < ARRAY_SIZE(non_runtime_cpregs); i++) {
+        const CPRegStateLevel *l = &non_runtime_cpregs[i];
+        if (l->regidx == regidx) {
+            return l->level;
+        }
+    }
+
+    return KVM_PUT_RUNTIME_STATE;
+}
+
 #define AARCH64_CORE_REG(x)   (KVM_REG_ARM64 | KVM_REG_SIZE_U64 | \
                  KVM_REG_ARM_CORE | KVM_REG_ARM_CORE_REG(x))
 
@@ -280,7 +308,7 @@ int kvm_arch_put_registers(CPUState *cs, int level)
         return ret;
     }
 
-    if (!write_list_to_kvmstate(cpu)) {
+    if (!write_list_to_kvmstate(cpu, level)) {
         return EINVAL;
     }