Re: VMX: x86: handle host TSC calibration failure

Gleb Natapov
2013-03-12T11:18:53Z

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.