We must also handle the reverse condition; TSC can't go backwards
when trapping, and it's possible that bad hardware offsetting
makes this problem visible when entering trapping mode.
This is accomodated by adding a 'bump' field to the computed
TSC; it's not pleasant but it works.
Signed-off-by: Zachary Amsden <zamsden@xxxxxxxxxx>
---
arch/x86/include/asm/kvm_host.h | 2 +
arch/x86/kvm/x86.c | 58 +++++++++++++++++++++++++++++++++++---
2 files changed, 55 insertions(+), 5 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 64569b0..950537c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -413,6 +413,8 @@ struct kvm_arch {
u32 virtual_tsc_khz;
u32 virtual_tsc_mult;
s8 virtual_tsc_shift;
+ s64 tsc_bump;
+ s64 last_tsc_bump_ns;
struct kvm_xen_hvm_config xen_hvm_config;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 33cb0f0..86f182a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -917,13 +917,48 @@ static void kvm_arch_set_tsc_khz(struct kvm *kvm, u32 this_tsc_khz)
static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
{
+ struct kvm_arch *arch = &vcpu->kvm->arch;
u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.last_tsc_nsec,
- vcpu->kvm->arch.virtual_tsc_mult,
- vcpu->kvm->arch.virtual_tsc_shift);
+ arch->virtual_tsc_mult,
+ arch->virtual_tsc_shift);
tsc += vcpu->arch.last_tsc_write;
+ if (unlikely(arch->tsc_bump)) {
+ s64 bump;
+
+ /*
+ * Ugh. There were a TSC bump. See how much time elapsed
+ * in cycles since last read, take it off the bump, but
+ * ensure TSC advances by at least one. We're serialized
+ * by the TSC write lock until the bump is gone.
+ */
+ spin_lock(&arch->tsc_write_lock);
+ bump = pvclock_scale_delta(kernel_ns - arch->last_tsc_bump_ns,
+ arch->virtual_tsc_mult,
+ arch->virtual_tsc_shift);
+ bump = arch->tsc_bump - bump + 1;
+ if (bump < 0) {
+ pr_debug("kvm: vpu%d zeroed TSC bump\n", vcpu->vcpu_id);
+ bump = 0;
+ }
+ arch->tsc_bump = bump;
+ arch->last_tsc_bump_ns = kernel_ns;
+ spin_unlock(&arch->tsc_write_lock);
+
+ tsc += bump;
+ }
return tsc;
}
+static void bump_guest_tsc(struct kvm_vcpu *vcpu, s64 bump, s64 kernel_ns)
+{
+ struct kvm *kvm = vcpu->kvm;
+ spin_lock(&kvm->arch.tsc_write_lock);
+ kvm->arch.tsc_bump += bump;
+ kvm->arch.last_tsc_bump_ns = kernel_ns;
+ spin_unlock(&vcpu->kvm->arch.tsc_write_lock);
+ pr_debug("kvm: vcpu%d bumped TSC by %lld\n", vcpu->vcpu_id, bump);
+}
+
void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
{
struct kvm *kvm = vcpu->kvm;
@@ -996,7 +1031,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
void *shared_kaddr;
unsigned long this_tsc_khz;
s64 kernel_ns, max_kernel_ns;
- u64 tsc_timestamp;
+ u64 tsc_timestamp, tsc;
bool kvmclock = (vcpu->time_page != NULL);
bool catchup = !kvmclock;
@@ -1035,7 +1070,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
}
if (catchup) {
- u64 tsc = compute_guest_tsc(v, kernel_ns);
+ tsc = compute_guest_tsc(v, kernel_ns);
if (tsc > tsc_timestamp)
kvm_x86_ops->adjust_tsc_offset(v, tsc-tsc_timestamp);
}
@@ -1048,8 +1083,21 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
if (!kvmclock) {
/* Now, see if we need to switch into trap mode */
if ((vcpu->tsc_mode == TSC_MODE_TRAP || vcpu->tsc_overrun) &&
- !vcpu->tsc_trapping)
+ !vcpu->tsc_trapping) {
+ /*
+ * Check for the (hopefully) unlikely event of the
+ * computed virtual TSC being before the TSC we were
+ * passing through in hardware. This can happen if
+ * the kernel has miscomputed tsc_khz, we miss an
+ * overrun condition, or via bad SMP calibration.
+ * If this is the case, we must add a bump to the
+ * virtual TSC; this suck.
+ */
+ if (unlikely(tsc < vcpu->last_guest_tsc))
+ bump_guest_tsc(v, vcpu->last_guest_tsc - tsc,
+ kernel_ns);
kvm_x86_ops->set_tsc_trap(v, 1);
+ }
/* If we're falling behind and not trapping, re-trigger */
if (!vcpu->tsc_trapping &&
--
1.7.1
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