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KVM: x86: Fix a possible backwards warp of kvmclock

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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>
This commit is contained in:
Zachary Amsden 2010-08-19 22:07:30 -10:00 committed by Avi Kivity
parent 347bb4448c
commit 1d5f066e0b
2 changed files with 44 additions and 2 deletions
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2
arch/x86/include/asm/kvm_host.h
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@ -339,6 +339,8 @@ struct kvm_vcpu_arch {
unsigned int time_offset;
struct page *time_page;
u64 last_host_tsc;
u64 last_guest_tsc;
u64 last_kernel_ns;
bool nmi_pending;
bool nmi_injected;

44
arch/x86/kvm/x86.c
View File

@ -55,6 +55,7 @@
#include <asm/mce.h>
#include <asm/i387.h>
#include <asm/xcr.h>
#include <asm/pvclock.h>
#define MAX_IO_MSRS 256
#define CR0_RESERVED_BITS \
@ -976,14 +977,15 @@ static int kvm_write_guest_time(struct kvm_vcpu *v)
struct kvm_vcpu_arch *vcpu = &v->arch;
void *shared_kaddr;
unsigned long this_tsc_khz;
s64 kernel_ns;
s64 kernel_ns, max_kernel_ns;
u64 tsc_timestamp;
if ((!vcpu->time_page))
return 0;
/* 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);
kernel_ns = get_kernel_ns();
this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
local_irq_restore(flags);
@ -993,13 +995,49 @@ static int kvm_write_guest_time(struct kvm_vcpu *v)
return 1;
}
/*
* 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 (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) {
kvm_set_time_scale(this_tsc_khz, &vcpu->hv_clock);
vcpu->hw_tsc_khz = this_tsc_khz;
}
if (max_kernel_ns > kernel_ns)
kernel_ns = max_kernel_ns;
/* With all the info we got, fill in the values */
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;
vcpu->hv_clock.flags = 0;
/*
@ -4931,6 +4969,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
if (hw_breakpoint_active())
hw_breakpoint_restore();
kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
atomic_set(&vcpu->guest_mode, 0);
smp_wmb();
local_irq_enable();