Re: [patch 7/8] Dont mark TSC unstable due to S4 suspend

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Feb 08, 2012 at 04:18:48PM +0100, Joerg Roedel wrote:
> On Fri, Feb 03, 2012 at 03:43:56PM -0200, Marcelo Tosatti wrote:
> > +	if (backwards_tsc) {
> > +		u64 delta_cyc = max_tsc - local_tsc;
> > +		list_for_each_entry(kvm, &vm_list, vm_list) {
> > +			kvm_for_each_vcpu(i, vcpu, kvm) {
> > +				vcpu->arch.tsc_offset_adjustment += delta_cyc;
> > +				vcpu->arch.last_host_tsc = local_tsc;
> > +			}
> > +
> > +			/*
> > +			 * We have to disable TSC offset matching.. if you were
> > +			 * booting a VM while issuing an S4 host suspend....
> > +			 * you may have some problem.  Solving this issue is
> > +			 * left as an exercise to the reader.
> > +			 */
> > +			kvm->arch.last_tsc_nsec = 0;
> > +			kvm->arch.last_tsc_write = 0;
> > +		}
> > +
> > +	}
> > +	return 0;
> 
> This is not going to work when tsc-scaling is enabled. The
> adjust_tsc_offset_host() function just scales the offset the same way
> the tsc is scaled. But that is broken because the tsc-offset is applied
> _after_ the tsc-ratio by scaling hardware. So to get the desired
> tsc-value in the guest the offset needs to be scaled in the opposite
> direction as the tsc itself. This is rather complicated to implement.

By saying that "tsc-offset is applied _after_ the tsc-ratio by scaling
hardware" you mean that guest tsc is calculated as

	tsc = host_tsc_value * tsc_ratio
	tsc += tsc_offset

?

If so, that means the tsc_offset must be scaled to guest tsc units.

Which is what both adjust_tsc_offset(host=true) and compute_tsc_offset()
do.

What am i missing here?

> I suggest to drop patch 6/8 and just let adjust_tsc_offset() be using
> guest-tsc units. These loops have to be changed then to work with the
> guest-tsc instead of the host-tsc.
> 
> 
> 	Joerg
> 
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