[PATCH] target/i386: introduce CPU property to work around Windows reset bug

[Paolo Bonzini]

Some versions of Windows hang on reboot if their TSC value is greater
than 2^54.  The calibration of the Hyper-V reference time overflows
and fails; as a result the processors' clock sources are out of sync.
As a workaround, reset the TSC to a small value.  Do not do this
unconditionally and require a special property to be set.

Reported-by: Vadim Rozenfeld <vrozenfe@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/cpu.c | 10 ++++++++++
 target/i386/cpu.h |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index ec3b50bf6e..9b29cea8c4 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -5931,6 +5931,15 @@ static void x86_cpu_reset(DeviceState *dev)
     env->xstate_bv = 0;
 
     env->pat = 0x0007040600070406ULL;
+
+    /*
+     * Some versions of Windows hang on reboot if their TSC value is greater
+     * than 2^54.  As a workaround, reset the TSC to a small value.  Do not use
+     * zero, KVM applies special heuristics for CPU startup when TSC is 
cleared.
+     */
+    if (cpu->tsc_clear_on_reset) {
+        env->tsc = 1;
+    }
     env->msr_ia32_misc_enable = MSR_IA32_MISC_ENABLE_DEFAULT;
     if (env->features[FEAT_1_ECX] & CPUID_EXT_MONITOR) {
         env->msr_ia32_misc_enable |= MSR_IA32_MISC_ENABLE_MWAIT;
@@ -6957,6 +6966,7 @@ static Property x86_cpu_properties[] = {
                      false),
     DEFINE_PROP_BOOL("vmware-cpuid-freq", X86CPU, vmware_cpuid_freq, true),
     DEFINE_PROP_BOOL("tcg-cpuid", X86CPU, expose_tcg, true),
+    DEFINE_PROP_BOOL("tsc-clear-on-reset", X86CPU, tsc_clear_on_reset, true),
     DEFINE_PROP_BOOL("x-migrate-smi-count", X86CPU, migrate_smi_count,
                      true),
     /*
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index e31e6bd8b8..66f7901729 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1864,6 +1864,9 @@ struct ArchCPU {
     /* Forcefully disable KVM PV features not exposed in guest CPUIDs */
     bool kvm_pv_enforce_cpuid;
 
+    /* Clear TSC on reset */
+    bool tsc_clear_on_reset;
+
     /* Number of physical address bits supported */
     uint32_t phys_bits;
 
-- 
2.35.1