On Fri, May 12, 2017 at 04:13:23PM +0200, Radim Krčmář wrote:
> 2017-05-11 12:39-0300, Marcelo Tosatti:
> > On Wed, May 10, 2017 at 08:04:31PM +0200, Radim Krčmář wrote:
> > > 2017-05-03 10:43-0300, Marcelo Tosatti:
> > > and the important fix for kvm master clock is the move of
> > > kvm_gen_update_masterclock() before we read the time.
>
> > > The rest is just a minor optimization that also ignores time since
> > > master_kernel_ns() and therefore pins user_ns.clock to a slightly
> > > earlier time.
> > >
> > > But all attention was given to the "minor optimization" -- have I missed
> > > something about the direct use of ka->master_kernel_ns?
> >
> > I haven't attempted to optimize anything. Not sure what you mean.
>
> I mean, why doesn't the patch look like this?
d
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 464da936c53d..8db1d09e59d7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4175,9 +4175,10 @@ long kvm_arch_vm_ioctl(struct file *filp,
> goto out;
>
> r = 0;
> + kvm_gen_update_masterclock(kvm);
> now_ns = get_kvmclock_ns(kvm);
> kvm->arch.kvmclock_offset += user_ns.clock - now_ns;
> - kvm_gen_update_masterclock(kvm);
> + kvm_make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE);
> break;
now_ns = ka->master_kernel_ns + kvmclock_offset_prev + (grdtsc() - ka->master_cycle_now)
kvmclock_offset += user_ns.clock - (ka->master_kernel_ns + kvmclock_offset_prev + (grdtsc() - ka->master_cycle_now)
kvmclock_offset = kvmclock_offset_prev + user_ns.clock - (ka->master_kernel_ns + kvmclock_offset_prev + (grdtsc() - ka->master_cycle_now)
In case of VM was just initialized before migration, kvmclock_offset_prev is -ktime_get_boot_ns()
kvmclock_offset = -ktime_get_boot_ns() + user_ns.clock - (ka->master_kernel_ns -ktime_get_boot_ns() + grdtsc() - ka->master_cycle_now))
But master_kernel_ns = ktime_get_boot_ns() + delta-between-vm-init-and-KVM_SET_CLOCK (AKA delta)
(the same one from VM init)
kvmclock_offset = -ktime_get_boot_ns() + user_ns.clock - (ktime_get_boot_ns() + delta + -ktime_get_boot_ns() + grdtsc() - ka->master_cycle_now))
kvmclock_offset = -ktime_get_boot_ns() + user_ns.clock - delta - grdtsc() + ka->master_cycle_now
But we don't want grdtsc() - ka->master_cycle_now in there.
Note: grdtsc() == guest read tsc.
Now with
+ kvm->arch.kvmclock_offset = user_ns.clock -
+ ka->master_kernel_ns;
What happens is that guest clock starts counting, via kernel timekeeper,
at the moment kvm_get_time_and_clockread() runs. If you add grdtsc() -
ka->master_cycle_now in there, you are mindfully counting clock twice
(first: kernel timekeeper, second: the TSC between the (grdtsc() -
ka->master_cycle_now) in question.
+ kvm->arch.kvmclock_offset = -ktime_get_boot_ns() +user_ns.clock -delta
Note that (grdtsc() - ka->master_cycle_now) is susceptible to scheduling
etc.
Makes sense?
