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

[Vadim Rozenfeld <vrozenfe@redhat.com>]

----- Original Message -----
From: "Paolo Bonzini" <pbonzini@redhat.com>
To: "Vadim Rozenfeld" <vrozenfe@redhat.com>
Cc: kvm@vger.kernel.org, gleb@redhat.com, mtosatti@redhat.com, pl@dlh.net
Sent: Friday, May 24, 2013 2:44:50 AM
Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC

Il 19/05/2013 09:06, Vadim Rozenfeld ha scritto:
> 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;

Thinking more of migration, could we increment whatever sequence value
we found (or better, do (x|3)+2 to skip over 0 and 0xFFFFFFFF), instead
of forcing it to 1?

[VR]
Yes, it should work.
We need to keep sequence between 1 and 0xFFFFFFFF and increment it every time
the VM was migrated or paused/resumed.

Add HV_X64_MSR_REFERENCE_TSC to msrs_to_save, and migration should just
work.

Paolo

> +		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);
>