[PATCH 10/18] Keep SMP VMs more in sync on unstable TSC

Zachary Amsden
2010-07-13T02:25:30Z

SMP VMs on machines with unstable TSC have their TSC offset adjusted by the
local offset delta from last measurement. This does not take into account how
long it has been since the measurement, leading to drift. Minimize the drift
by accounting for any time difference the kernel has observed.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/x86.c | 8 +++++++-
2 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 324e892..d9e0aa0 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -339,6 +339,7 @@ struct kvm_vcpu_arch {
unsigned int time_offset;
struct page *time_page;
u64 last_host_tsc;
+ u64 last_host_ns;

bool nmi_pending;
bool nmi_injected;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 51d3f3e..a4d7cb9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1888,8 +1888,13 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
native_read_tsc() - vcpu->arch.last_host_tsc;
if (tsc_delta < 0)
mark_tsc_unstable("KVM discovered backwards TSC");
- if (check_tsc_unstable())
+ if (check_tsc_unstable()) {
+ /* Subtract elapsed cycle time */
+ u64 ns = !vcpu->arch.last_host_ns ? 0 :
+ get_kernel_ns() - vcpu->arch.last_host_ns;
+ tsc_delta -= nsec_to_cycles(ns);
kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
+ }
kvm_migrate_timers(vcpu);
kvm_request_guest_time_update(vcpu);
vcpu->cpu = cpu;
@@ -1901,6 +1906,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
kvm_x86_ops->vcpu_put(vcpu);
kvm_put_guest_fpu(vcpu);
vcpu->arch.last_host_tsc = native_read_tsc();
+ vcpu->arch.last_host_ns = get_kernel_ns();
}

static int is_efer_nx(void)
--
1.7.1