On Mon, Nov 14, 2016 at 03:52:40PM +0100, Radim Krčmář wrote:
> 2016-11-11 11:12+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.
> >
> > This is easily fixed however, because kvm_get_time_and_clockread provides
> > the information we want.
> >
> > Reported-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.c | 18 ++++++++++--------
> > 1 file changed, 10 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 1ba08278a9a9..1c16c6d7df7a 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1620,6 +1620,11 @@ static bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
> >
> > return do_monotonic_boot(kernel_ns, cycle_now) == VCLOCK_TSC;
> > }
> > +#else
> > +static inline bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
> > +{
> > + return false;
> > +}
> > #endif
> >
> > /*
> > @@ -1724,18 +1729,15 @@ 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;
> > + cycle_t cycle_now;
> > s64 ns;
> >
> > - 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);
>
> This patch regresses the behavior as well, because the assumption that
> kvm_get_time_and_clockread() and __pvclock_read_cycles() count the same
> time doesn't hold. See the end of the message for a quick test.
>
> kvm_get_time_and_clockread() is actually the same as ktime_get_boot_ns()
> (if it works), so we'd be just obfucating the code. :)
>
> I think that making kvmclock count as ktime_get_boot_ns() would be the
> best solution, but not possible this late in 4.9 ...
>
> As a quick hack, I think it would be better to duplicate the update that
> would happen when running the VCPU before calling
> __pvclock_read_cycles(), i.e. paste something like this:
>
> if (kvm_check_request(KVM_REQ_CLOCK_UPDATE, vcpu))
> kvm_guest_time_update(vcpu);
>
> > - } else {
> > - ns = ktime_get_boot_ns() + ka->kvmclock_offset;
> > - }
> > + if (!ka->use_master_clock ||
> > + !kvm_get_time_and_clockread(&ns, &cycle_now))
> > + ns = ktime_get_boot_ns();
> >
> > - return ns;
> > + return ns + ka->kvmclock_offset;
> > }
>
> The hunk below should return the same value in pvclock_ns and kernel_ns
> if they can be used interchangeably. boot_ns is expected to be a bit
> delayed, because it is read late. boot_ns shows a bounded offset from
> kernel_ns, unlike the drifting pvclock_ns.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 83990ad3710e..30d4d3d02ac7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6653,6 +6653,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> goto cancel_injection;
> }
>
> + if (vcpu->kvm->arch.use_master_clock) {
> + s64 kernel_ns;
> + cycle_t tsc_now, pvclock_ns, boot_ns;
> +
> + kvm_get_time_and_clockread(&kernel_ns, &tsc_now);
> + pvclock_ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, kvm_read_l1_tsc(vcpu, tsc_now)) - vcpu->kvm->arch.kvmclock_offset;
> + boot_ns = ktime_get_boot_ns();
> +
> + printk("ns diff: %lld %lld\n", pvclock_ns - kernel_ns, boot_ns - kernel_ns);
> + }
> +
> preempt_disable();
>
> kvm_x86_ops->prepare_guest_switch(vcpu);
>
> and a sample output:
KVM_GET_CLOCK should return what the guest sees at the moment
KVM_GET_CLOCK is called, which should include
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;
>>> add (rdtsc() - tsc_timestamp),
if kvmclock is enabled
}
The addition under >>> above.
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