On Wed, May 22, 2013 at 06:23:30PM -0300, Marcelo Tosatti wrote:
> On Wed, May 22, 2013 at 03:22:55AM -0400, Vadim Rozenfeld wrote:
> >
> >
> > ----- Original Message -----
> > From: "Marcelo Tosatti" <[email protected]>
> > To: "Vadim Rozenfeld" <[email protected]>
> > Cc: [email protected], [email protected], [email protected]
> > 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?
>
Please read the whole discussion, we talked about it already. We
definitely do not want to fall back to PM timer either, we want to use
reference counter instead.
--
Gleb.
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