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

[Vadim Rozenfeld <vrozenfe@redhat.com>]

----- 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.

> +		tsc_ref.TscScale =
> +				((10000LL << 32) / vcpu->arch.virtual_tsc_khz) << 32;
> +		tsc_ref.TscOffset = 0;
>  		if (__copy_to_user((void __user *)addr, &tsc_ref, sizeof(tsc_ref)))
>  			return 1;
>  		mark_page_dirty(kvm, gfn);
> -- 
> 1.8.1.2