[Qemu-devel] [PULL 3/9] target-i386: clear guest TSC on reset

[Paolo Bonzini]

From: Fernando Luis Vázquez Cao <address@hidden>

VCPU TSC is not cleared by a warm reset (*), which leaves some types of Linux
 guests (non-pvops guests and those with the kernel parameter no-kvmclock set)
vulnerable to the overflow in cyc2ns_offset fixed by upstream commit
9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 ("sched/x86: Fix overflow in
cyc2ns_offset").

To put it in a nutshell, if such a Linux guest without the patch above applied
has been up more than 208 days and attempts a warm reset chances are that
the newly booted kernel will panic or hang.

(*) Intel Xeon E5 processors show the same broken behavior due to
    the errata "TSC is Not Affected by Warm Reset" (Intel® Xeon®
    Processor E5 Family Specification Update - August 2013): "The
    TSC (Time Stamp Counter MSR 10H) should be cleared on
    reset. Due to this erratum the TSC is not affected by warm
    reset."

Cc: Will Auld <address@hidden>
Cc: Marcelo Tosatti <address@hidden>
Signed-off-by: Fernando Luis Vazquez Cao <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
Signed-off-by: Fernando Luis Vázquez Cao <address@hidden>
---
 target-i386/cpu.c | 3 +++
 target-i386/kvm.c | 4 +---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 5076a94..bc4cb9d 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -2450,6 +2450,9 @@ static void x86_cpu_reset(CPUState *s)
     cpu_breakpoint_remove_all(env, BP_CPU);
     cpu_watchpoint_remove_all(env, BP_CPU);
 
+    env->tsc_adjust = 0;
+    env->tsc = 0;
+
 #if !defined(CONFIG_USER_ONLY)
     /* We hard-wire the BSP to the first CPU. */
     if (s->cpu_index == 0) {
diff --git a/target-i386/kvm.c b/target-i386/kvm.c
index 312a46b..285e1a3 100644
--- a/target-i386/kvm.c
+++ b/target-i386/kvm.c
@@ -1150,14 +1150,12 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
         kvm_msr_entry_set(&msrs[n++], MSR_LSTAR, env->lstar);
     }
 #endif
-    if (level == KVM_PUT_FULL_STATE) {
-        kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
-    }
     /*
      * The following MSRs have side effects on the guest or are too heavy
      * for normal writeback. Limit them to reset or full state updates.
      */
     if (level >= KVM_PUT_RESET_STATE) {
+        kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
         kvm_msr_entry_set(&msrs[n++], MSR_KVM_SYSTEM_TIME,
                           env->system_time_msr);
         kvm_msr_entry_set(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
-- 
1.8.3.1