Date: Tue, 15 Jun 2010 21:27:05 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Zachary Amsden <zamsden@redhat.com>
Cc: avi@redhat.com, glommer@redhat.com, kvm@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 16/17] TSC reset compensation
Message-ID: <20100616002705.GA15994@amt.cnet>
References: <1276587259-32319-1-git-send-email-zamsden@redhat.com>
 <1276587259-32319-17-git-send-email-zamsden@redhat.com>
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On Mon, Jun 14, 2010 at 09:34:18PM -1000, Zachary Amsden wrote:
> Attempt to synchronize TSCs which are reset to the same value.  In the
> case of a reliable hardware TSC, we can just re-use the same offset, but
> on non-reliable hardware, we can get closer by adjusting the offset to
> match the elapsed time.
> 
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>
> ---
>  arch/x86/kvm/x86.c |   34 ++++++++++++++++++++++++++++++++--
>  1 files changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8e836e9..cedb71f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v)
>  	set_bit(KVM_REQ_CLOCK_SYNC, &v->requests);
>  }
>  
> +static inline int kvm_tsc_reliable(void)
> +{
> +	return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> +		boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> +		!check_tsc_unstable());
> +}
> +
>  void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>  {
>  	struct kvm *kvm = vcpu->kvm;
> -	u64 offset;
> +	u64 offset, ns, elapsed;
>  
>  	spin_lock(&kvm->arch.tsc_write_lock);
>  	offset = data - native_read_tsc();
> -	kvm->arch.last_tsc_nsec = get_kernel_ns();
> +	ns = get_kernel_ns();
> +	elapsed = ns - kvm->arch.last_tsc_nsec;
> +
> +	/*
> +	 * Special case: identical write to TSC within 5 seconds of
> +	 * another CPU is interpreted as an attempt to synchronize
> +	 * (the 5 seconds is to accomodate host load / swapping).
> +	 *
> +	 * In that case, for a reliable TSC, we can match TSC offsets,
> +	 * or make a best guest using kernel_ns value.
> +	 */
> +	if (data == kvm->arch.last_tsc_write && elapsed < 5 * NSEC_PER_SEC) {
> +		if (kvm_tsc_reliable()) {
> +			offset = kvm->arch.last_tsc_offset;
> +			pr_debug("kvm: matched tsc offset for %llu\n", data);
> +		} else {
> +			u64 tsc_delta = elapsed * __get_cpu_var(cpu_tsc_khz);
> +			tsc_delta = tsc_delta / USEC_PER_SEC;
> +			offset -= tsc_delta;
> +			pr_debug("kvm: adjusted tsc offset by %llu\n", tsc_delta);
> +		}
> +		ns = kvm->arch.last_tsc_nsec;
> +	}
> +	kvm->arch.last_tsc_nsec = ns;
>  	kvm->arch.last_tsc_write = data;
>  	kvm->arch.last_tsc_offset = offset;
>  	kvm_x86_ops->write_tsc_offset(vcpu, offset);
> -- 

Could extend this to handle migration.
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