Before commit 78db6a5037965429c04d708281f35a6e5562d31b,
kvm_guest_time_update() would use vcpu->virtual_tsc_khz to calculate
tsc_shift value in the vcpus pvclock structure written to guest memory.
For those kernels, if vcpu->virtual_tsc_khz != tsc_khz (which can be the
case when guest state is restored via migration, or if tsc-khz option is
passed to QEMU), and TSC scaling is not enabled (which happens if the
difference between the frequency requested via KVM_SET_TSC_KHZ and the
host TSC KHZ is smaller than 250ppm), then there can be a difference
between what KVM_GET_CLOCK would return and what the guest reads as
kvmclock value.
When KVM_SET_CLOCK'ing what is read with KVM_GET_CLOCK, the
guest can observe a forward or backwards time jump.
Advertise to userspace that current kernel contains
this fix, so QEMU can workaround the problem by reading
pvclock via guest memory directly otherwise.
Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6aaae18f1854..61c5876cfd87 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2160,7 +2160,8 @@ static inline int kvm_cpu_get_apicid(int mps_cpu)
int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
#define KVM_CLOCK_VALID_FLAGS \
- (KVM_CLOCK_TSC_STABLE | KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC)
+ (KVM_CLOCK_TSC_STABLE | KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC | \
+ KVM_CLOCK_CORRECT_TSC_SHIFT)
#define KVM_X86_VALID_QUIRKS \
(KVM_X86_QUIRK_LINT0_REENABLED | \
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index da4bbd043a7b..142a53d06100 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3014,6 +3014,8 @@ static void __get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
}
+ data->flags |= KVM_CLOCK_CORRECT_TSC_SHIFT;
+
put_cpu();
}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 55155e262646..7b94d7bd03a6 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1301,6 +1301,9 @@ struct kvm_irqfd {
#define KVM_CLOCK_TSC_STABLE 2
#define KVM_CLOCK_REALTIME (1 << 2)
#define KVM_CLOCK_HOST_TSC (1 << 3)
+/* whether tsc_shift as seen by the guest matches guest visible TSC */
+/* This is true since commit 78db6a5037965429c04d708281f35a6e5562d31b */
+#define KVM_CLOCK_CORRECT_TSC_SHIFT (1 << 4)
struct kvm_clock_data {
__u64 clock;
