Commit 26769f96 authored Jun 16, 2020 by Marcelo Tosatti Committed by Paolo Bonzini Jun 23, 2020

KVM: x86: allow TSC to differ by NTP correction bounds without TSC scaling



The Linux TSC calibration procedure is subject to small variations
(its common to see +-1 kHz difference between reboots on a given CPU, for example).

So migrating a guest between two hosts with identical processor can fail, in case
of a small variation in calibrated TSC between them.

Without TSC scaling, the current kernel interface will either return an error
(if user_tsc_khz <= tsc_khz) or enable TSC catchup mode.

This change enables the following TSC tolerance check to
accept KVM_SET_TSC_KHZ within tsc_tolerance_ppm (which is 250ppm by default).

        /*
         * Compute the variation in TSC rate which is acceptable
         * within the range of tolerance and decide if the
         * rate being applied is within that bounds of the hardware
         * rate.  If so, no scaling or compensation need be done.
         */
        thresh_lo = adjust_tsc_khz(tsc_khz, -tsc_tolerance_ppm);
        thresh_hi = adjust_tsc_khz(tsc_khz, tsc_tolerance_ppm);
        if (user_tsc_khz < thresh_lo || user_tsc_khz > thresh_hi) {
                pr_debug("kvm: requested TSC rate %u falls outside tolerance [%u,%u]\n", user_tsc_khz, thresh_lo, thresh_hi);
                use_scaling = 1;
        }

NTP daemon in the guest can correct this difference (NTP can correct upto 500ppm).

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Message-Id: <20200616114741.GA298183@fuller.cnet>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent bf10bd0b

arch/x86/kvm/x86.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -4603,7 +4603,8 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
		r = -EINVAL;
		user_tsc_khz = (u32)arg;

		if (user_tsc_khz >= kvm_max_guest_tsc_khz)
		if (kvm_has_tsc_control &&
		user_tsc_khz >= kvm_max_guest_tsc_khz)
		goto out;

		if (user_tsc_khz == 0)

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