Re: VMX: x86: handle host TSC calibration failure

[Gleb Natapov <gleb@redhat.com>]

On Mon, Mar 11, 2013 at 11:10:24PM -0300, Marcelo Tosatti wrote:
> 
> If the host TSC calibration fails, tsc_khz is zero (see tsc_init.c).
> Handle such case properly in KVM (instead of dividing by zero).
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=859282
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
Fixed small space/tab problem and applied. Thanks.

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 35b4912..19741b9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1079,6 +1079,10 @@ static void kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 this_tsc_khz)
>  	u32 thresh_lo, thresh_hi;
>  	int use_scaling = 0;
>  
> +	/* tsc_khz can be zero if TSC calibration fails */
> +	if (this_tsc_khz == 0)
> +		return;
> +
>  	/* Compute a scale to convert nanoseconds in TSC cycles */
>  	kvm_get_time_scale(this_tsc_khz, NSEC_PER_SEC / 1000,
>  			   &vcpu->arch.virtual_tsc_shift,
> @@ -1156,20 +1160,23 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  	ns = get_kernel_ns();
>  	elapsed = ns - kvm->arch.last_tsc_nsec;
>  
> -	/* n.b - signed multiplication and division required */
> -	usdiff = data - kvm->arch.last_tsc_write;
> +	if (vcpu->arch.virtual_tsc_khz) {
> +		/* n.b - signed multiplication and division required */
> +		usdiff = data - kvm->arch.last_tsc_write;
>  #ifdef CONFIG_X86_64
> -	usdiff = (usdiff * 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"(usdiff)
> -	    : "A"(usdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
> +		/* do_div() only does unsigned */
> +		asm("idivl %2; xor %%edx, %%edx"
> +	    	: "=A"(usdiff)
> +	    	: "A"(usdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));
>  #endif
> -	do_div(elapsed, 1000);
> -	usdiff -= elapsed;
> -	if (usdiff < 0)
> -		usdiff = -usdiff;
> +		do_div(elapsed, 1000);
> +		usdiff -= elapsed;
> +		if (usdiff < 0)
> +			usdiff = -usdiff;
> +	} else
> +		usdiff = USEC_PER_SEC; /* disable TSC match window below */
>  
>  	/*
>  	 * Special case: TSC write with a small delta (1 second) of virtual

--
			Gleb.