Going through the first VCPU is wrong if you follow a KVM_SET_CLOCK with
a KVM_GET_CLOCK immediately after, without letting the VCPU run and
call kvm_guest_time_update.
To fix this, compute the kvmclock value ourselves, using the master
clock (tsc, nsec) pair as the base and the host CPU frequency as
the scale.
Reported-by: Marcelo Tosatti <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
---
arch/x86/kvm/x86.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1ba08278a9a9..bd138a79404a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1724,18 +1724,23 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
static u64 __get_kvmclock_ns(struct kvm *kvm)
{
- struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
struct kvm_arch *ka = &kvm->arch;
- s64 ns;
+ struct pvclock_vcpu_time_info hv_clock;
- if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
- u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
- ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
- } else {
- ns = ktime_get_boot_ns() + ka->kvmclock_offset;
+ spin_lock(&ka->pvclock_gtod_sync_lock);
+ if (!ka->use_master_clock) {
+ spin_unlock(&ka->pvclock_gtod_sync_lock);
+ return ktime_get_boot_ns() + ka->kvmclock_offset;
}
- return ns;
+ hv_clock.tsc_timestamp = ka->master_cycle_now;
+ hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
+ spin_unlock(&ka->pvclock_gtod_sync_lock);
+
+ kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
+ &hv_clock.tsc_shift,
+ &hv_clock.tsc_to_system_mul);
+ return __pvclock_read_cycles(&hv_clock, rdtsc());
}
u64 get_kvmclock_ns(struct kvm *kvm)
--
1.8.3.1
2016-11-16 18:31+0100, Paolo Bonzini:
> Going through the first VCPU is wrong if you follow a KVM_SET_CLOCK with
> a KVM_GET_CLOCK immediately after, without letting the VCPU run and
> call kvm_guest_time_update.
>
> To fix this, compute the kvmclock value ourselves, using the master
> clock (tsc, nsec) pair as the base and the host CPU frequency as
> the scale.
>
> Reported-by: Marcelo Tosatti <[email protected]>
> Signed-off-by: Paolo Bonzini <[email protected]>
> ---
Looks good, thanks! Planned for 4.9-rc6.
On Wed, Nov 16, 2016 at 06:31:30PM +0100, Paolo Bonzini wrote:
> Going through the first VCPU is wrong if you follow a KVM_SET_CLOCK with
> a KVM_GET_CLOCK immediately after, without letting the VCPU run and
> call kvm_guest_time_update.
>
> To fix this, compute the kvmclock value ourselves, using the master
> clock (tsc, nsec) pair as the base and the host CPU frequency as
> the scale.
>
> Reported-by: Marcelo Tosatti <[email protected]>
> Signed-off-by: Paolo Bonzini <[email protected]>
> ---
> arch/x86/kvm/x86.c | 21 +++++++++++++--------
> 1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1ba08278a9a9..bd138a79404a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1724,18 +1724,23 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
>
> static u64 __get_kvmclock_ns(struct kvm *kvm)
> {
> - struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
> struct kvm_arch *ka = &kvm->arch;
> - s64 ns;
> + struct pvclock_vcpu_time_info hv_clock;
>
> - if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
> - u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> - ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
> - } else {
> - ns = ktime_get_boot_ns() + ka->kvmclock_offset;
> + spin_lock(&ka->pvclock_gtod_sync_lock);
> + if (!ka->use_master_clock) {
> + spin_unlock(&ka->pvclock_gtod_sync_lock);
> + return ktime_get_boot_ns() + ka->kvmclock_offset;
> }
>
> - return ns;
> + hv_clock.tsc_timestamp = ka->master_cycle_now;
> + hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
> + spin_unlock(&ka->pvclock_gtod_sync_lock);
> +
> + kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
> + &hv_clock.tsc_shift,
> + &hv_clock.tsc_to_system_mul);
> + return __pvclock_read_cycles(&hv_clock, rdtsc());
> }
Missing TSC scaling?
/* With all the info we got, fill in the values */
if (kvm_has_tsc_control)
tgt_tsc_khz = kvm_scale_tsc(v, tgt_tsc_khz);
Should use kvm_read_l1_tsc to convert as well?
On 17/11/2016 12:38, Marcelo Tosatti wrote:
> On Wed, Nov 16, 2016 at 06:31:30PM +0100, Paolo Bonzini wrote:
>> Going through the first VCPU is wrong if you follow a KVM_SET_CLOCK with
>> a KVM_GET_CLOCK immediately after, without letting the VCPU run and
>> call kvm_guest_time_update.
>>
>> To fix this, compute the kvmclock value ourselves, using the master
>> clock (tsc, nsec) pair as the base and the host CPU frequency as
>> the scale.
>>
>> Reported-by: Marcelo Tosatti <[email protected]>
>> Signed-off-by: Paolo Bonzini <[email protected]>
>> ---
>> arch/x86/kvm/x86.c | 21 +++++++++++++--------
>> 1 file changed, 13 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 1ba08278a9a9..bd138a79404a 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1724,18 +1724,23 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
>>
>> static u64 __get_kvmclock_ns(struct kvm *kvm)
>> {
>> - struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
>> struct kvm_arch *ka = &kvm->arch;
>> - s64 ns;
>> + struct pvclock_vcpu_time_info hv_clock;
>>
>> - if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
>> - u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>> - ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
>> - } else {
>> - ns = ktime_get_boot_ns() + ka->kvmclock_offset;
>> + spin_lock(&ka->pvclock_gtod_sync_lock);
>> + if (!ka->use_master_clock) {
>> + spin_unlock(&ka->pvclock_gtod_sync_lock);
>> + return ktime_get_boot_ns() + ka->kvmclock_offset;
>> }
>>
>> - return ns;
>> + hv_clock.tsc_timestamp = ka->master_cycle_now;
>> + hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
>> + spin_unlock(&ka->pvclock_gtod_sync_lock);
>> +
>> + kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>> + &hv_clock.tsc_shift,
>> + &hv_clock.tsc_to_system_mul);
>> + return __pvclock_read_cycles(&hv_clock, rdtsc());
>> }
>
> Missing TSC scaling?
>
> /* With all the info we got, fill in the values */
>
> if (kvm_has_tsc_control)
> tgt_tsc_khz = kvm_scale_tsc(v, tgt_tsc_khz);
>
> Should use kvm_read_l1_tsc to convert as well?
We don't have a vcpu, so we cannot use either kvm_scale_tsc or
kvm_read_l1_tsc. But luckily it's not necessary here, because we're
using cpu_tsc_khz and rdtsc.
Paolo