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

[Gleb Natapov <gleb@redhat.com>]

On Thu, May 23, 2013 at 08:18:55AM +0200, Peter Lieven wrote:
> On 22.05.2013 23:23, Marcelo Tosatti wrote:
> >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?
> >
> >
> 
> what exactly do we heed the reference TSC for? the reference counter alone works great and it seems
> that there is a lot of trouble and crash possibilities involved with the referece tsc.
> 
Reference TSC is even faster. There should be no crashed with proper
implementation.

--
			Gleb.