Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, May 22, 2013 at 03:22:55AM -0400, Vadim Rozenfeld wrote:
> 
> 
> ----- Original Message -----
> From: "Marcelo Tosatti" <mtosatti@redhat.com>
> To: "Vadim Rozenfeld" <vrozenfe@redhat.com>
> Cc: kvm@vger.kernel.org, gleb@redhat.com, pl@dlh.net
> Sent: Wednesday, May 22, 2013 10:50:46 AM
> Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC
> 
> On Sun, May 19, 2013 at 05:06:37PM +1000, Vadim Rozenfeld wrote:
> > The following patch allows to activate a partition reference 
> > time enlightenment that is based on the host platform's support 
> > for an Invariant Time Stamp Counter (iTSC).
> > NOTE: This code will survive migration due to lack of VM stop/resume
> > handlers, when offset, scale and sequence should be
> > readjusted. 
> > 
> > ---
> >  arch/x86/kvm/x86.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 9645dab..b423fe4 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1838,7 +1838,6 @@ static int set_msr_hyperv_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data)
> >  		u64 gfn;
> >  		unsigned long addr;
> >  		HV_REFERENCE_TSC_PAGE tsc_ref;
> > -		tsc_ref.TscSequence = 0;
> >  		if (!(data & HV_X64_MSR_TSC_REFERENCE_ENABLE)) {
> >  			kvm->arch.hv_tsc_page = data;
> >  			break;
> > @@ -1848,6 +1847,11 @@ static int set_msr_hyperv_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data)
> >  				HV_X64_MSR_TSC_REFERENCE_ADDRESS_SHIFT);
> >  		if (kvm_is_error_hva(addr))
> >  			return 1;
> > +		tsc_ref.TscSequence =
> > +				boot_cpu_has(X86_FEATURE_CONSTANT_TSC) ? 1 : 0;
> 
> 1) You want NONSTOP_TSC (see 40fb1715 commit) which matches INVARIANT TSC.
> [VR]
> Thank you for reviewing. Will fix it.
> 2) TscSequence should increase? 
> "This field serves as a sequence number that is incremented whenever..."
> [VR]
> Yes, on every VM resume, including migration. After migration we also need
> to recalculate scale and adjust offset. 
> 3) 0xFFFFFFFF is the value for invalid source of reference time?
> [VR] Yes, on boot-up. In this case guest will go with PMTimer (not sure about HPET
> but I can check). But if we set sequence to 0xFFFFFFFF after migration - it's probably will not work.

"Reference TSC during Save and Restore and Migration

To address migration scenarios to physical platforms that do not support
iTSC, the TscSequence field is used. In the event that a guest partition
is  migrated from an iTSC capable host to a non-iTSC capable host, the
hypervisor sets TscSequence to the special value of 0xFFFFFFFF, which
directs the guest operating system to fall back to a different clock
source (for example, the virtual PM timer)."

Why it would not/does not work after migration?