Re: KVM: x86: fix kvm_write_tsc() TSC matching thinko

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-03-08 22:46, Marcelo Tosatti wrote:
> 
> kvm_write_tsc() converts from guest TSC to microseconds, not nanoseconds
> as intended. The result is that the window for matching is 1000 seconds,
> not 1 second.
> 
> Microsecond precision is enough for checking whether the TSC write delta
> is within the heuristic values, so use it instead of nanoseconds.
> 
> Noted by Avi Kivity.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 32096cf..9732e98 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1025,7 +1025,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>  	struct kvm *kvm = vcpu->kvm;
>  	u64 offset, ns, elapsed;
>  	unsigned long flags;
> -	s64 nsdiff;
> +	s64 usdiff;
>  
>  	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
>  	offset = kvm_x86_ops->compute_tsc_offset(vcpu, data);
> @@ -1033,18 +1033,18 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>  	elapsed = ns - kvm->arch.last_tsc_nsec;
>  
>  	/* n.b - signed multiplication and division required */
> -	nsdiff = data - kvm->arch.last_tsc_write;
> +	usdiff = data - kvm->arch.last_tsc_write;
>  #ifdef CONFIG_X86_64
> -	nsdiff = (nsdiff * 1000) / vcpu->arch.virtual_tsc_khz;
> +	usdiff = (usdiff * 1000) / vcpu->arch.virtual_tsc_khz;
>  #else
>  	/* do_div() only does unsigned */
>  	asm("idivl %2; xor %%edx, %%edx"
> -	    : "=A"(nsdiff)
> -	    : "A"(nsdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
> +	    : "=A"(usdiff)
> +	    : "A"(usdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
>  #endif
> -	nsdiff -= elapsed;
> -	if (nsdiff < 0)
> -		nsdiff = -nsdiff;
> +	usdiff -= elapsed / 1000;

do_div, to please x86-32.

Jan

> +	if (usdiff < 0)
> +		usdiff = -usdiff;
>  
>  	/*
>  	 * Special case: TSC write with a small delta (1 second) of virtual
> @@ -1056,7 +1056,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>  	 * compensation code attempt to catch up if we fall behind, but
>  	 * it's better to try to match offsets from the beginning.
>           */
> -	if (nsdiff < NSEC_PER_SEC &&
> +	if (usdiff < USEC_PER_SEC &&
>  	    vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
>  		if (!check_tsc_unstable()) {
>  			offset = kvm->arch.cur_tsc_offset;
> --
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