linux-kernel - Re: [PATCH 16/17] TSC reset compensation

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Date:	Tue, 15 Jun 2010 11:51:04 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	mtosatti@...hat.com, glommer@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] TSC reset compensation

On 06/15/2010 10:34 AM, 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.
>
>    

Answers a question from earlier.

I wonder about guests that might try to be clever an compensate for the 
IPI round trip, so not writing the same value.  On the other hand, 
really clever guests will synchronize though memory, not an IPI.

> Signed-off-by: Zachary Amsden<zamsden@...hat.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) {
>    

5e9 will overflow on i386.

> +		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;
>    

Shouldn't we check that the older write was on a different vcpu?

-- 
error compiling committee.c: too many arguments to function

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