Re: [PATCH v2] KVM: x86: fix KVM_SET_CLOCK relative to setting correct clock value

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-05-12 12:31-0300, Marcelo Tosatti:
> 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?

Yes.  The simpler code starts the kvmclock a bit later, but both are
correct -- anything within KVM_SET_CLOCK runtime is.

If we care about accuracy, then we should let userspace provide a
(kernel timestamp, kvm timestamp) pair, so the value of kvmclock can
really be controlled.

Adding ugly optimizations to work around shortcomings of the API is
going the wrong way ...