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Subject[142/143] KVM: x86: Fix a possible backwards warp of kvmclock
2.6.32-longterm review patch.  If anyone has any objections, please let us know.

------------------
From: Zachary Amsden <zamsden@redhat.com>

(backported from commit 1d5f066e0b63271b67eac6d3752f8aa96adcbddb)


Kernel time, which advances in discrete steps may progress much slower
than TSC. As a result, when kvmclock is adjusted to a new base, the
apparent time to the guest, which runs at a much higher, nsec scaled
rate based on the current TSC, may have already been observed to have
a larger value (kernel_ns + scaled tsc) than the value to which we are
setting it (kernel_ns + 0).

We must instead compute the clock as potentially observed by the guest
for kernel_ns to make sure it does not go backwards.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

BugLink: http://bugs.launchpad.net/bugs/714335

Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Reviewed-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
arch/x86/include/asm/kvm_host.h | 3 ++
arch/x86/kvm/x86.c | 47 +++++++++++++++++++++++++++++++++++++---
2 files changed, 47 insertions(+), 3 deletions(-)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -357,6 +357,9 @@ struct kvm_vcpu_arch {
struct page *time_page;

bool singlestep; /* guest is single stepped by KVM */
+ u64 last_guest_tsc;
+ u64 last_kernel_ns;
+
bool nmi_pending;
bool nmi_injected;

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -47,6 +47,7 @@
#include <asm/desc.h>
#include <asm/mtrr.h>
#include <asm/mce.h>
+#include <asm/pvclock.h>

#define MAX_IO_MSRS 256
#define CR0_RESERVED_BITS \
@@ -633,6 +634,8 @@ static void kvm_write_guest_time(struct
struct kvm_vcpu_arch *vcpu = &v->arch;
void *shared_kaddr;
unsigned long this_tsc_khz;
+ s64 kernel_ns, max_kernel_ns;
+ u64 tsc_timestamp;

if ((!vcpu->time_page))
return;
@@ -646,15 +649,51 @@ static void kvm_write_guest_time(struct

/* Keep irq disabled to prevent changes to the clock */
local_irq_save(flags);
- kvm_get_msr(v, MSR_IA32_TSC, &vcpu->hv_clock.tsc_timestamp);
+ kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
ktime_get_ts(&ts);
monotonic_to_bootbased(&ts);
+ kernel_ns = timespec_to_ns(&ts);
local_irq_restore(flags);

+ /*
+ * Time as measured by the TSC may go backwards when resetting the base
+ * tsc_timestamp. The reason for this is that the TSC resolution is
+ * higher than the resolution of the other clock scales. Thus, many
+ * possible measurments of the TSC correspond to one measurement of any
+ * other clock, and so a spread of values is possible. This is not a
+ * problem for the computation of the nanosecond clock; with TSC rates
+ * around 1GHZ, there can only be a few cycles which correspond to one
+ * nanosecond value, and any path through this code will inevitably
+ * take longer than that. However, with the kernel_ns value itself,
+ * the precision may be much lower, down to HZ granularity. If the
+ * first sampling of TSC against kernel_ns ends in the low part of the
+ * range, and the second in the high end of the range, we can get:
+ *
+ * (TSC - offset_low) * S + kns_old > (TSC - offset_high) * S + kns_new
+ *
+ * As the sampling errors potentially range in the thousands of cycles,
+ * it is possible such a time value has already been observed by the
+ * guest. To protect against this, we must compute the system time as
+ * observed by the guest and ensure the new system time is greater.
+ */
+ max_kernel_ns = 0;
+ if (vcpu->hv_clock.tsc_timestamp && vcpu->last_guest_tsc) {
+ max_kernel_ns = vcpu->last_guest_tsc -
+ vcpu->hv_clock.tsc_timestamp;
+ max_kernel_ns = pvclock_scale_delta(max_kernel_ns,
+ vcpu->hv_clock.tsc_to_system_mul,
+ vcpu->hv_clock.tsc_shift);
+ max_kernel_ns += vcpu->last_kernel_ns;
+ }
+
+ if (max_kernel_ns > kernel_ns)
+ kernel_ns = max_kernel_ns;
+
/* With all the info we got, fill in the values */

- vcpu->hv_clock.system_time = ts.tv_nsec +
- (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset;
+ vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
+ vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
+ vcpu->last_kernel_ns = kernel_ns;

/*
* The interface expects us to write an even number signaling that the
@@ -3695,6 +3734,8 @@ static int vcpu_enter_guest(struct kvm_v
kvm_x86_ops->prepare_guest_switch(vcpu);
kvm_load_guest_fpu(vcpu);

+ kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
+
local_irq_disable();

clear_bit(KVM_REQ_KICK, &vcpu->requests);



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