diff options
| author | Marcelo Tosatti | 2020-06-16 08:47:41 -0300 |
|---|---|---|
| committer | Paolo Bonzini | 2020-06-23 05:55:17 -0400 |
| commit | 26769f96e6231095f6b1cc3090c903280d44bb57 (patch) | |
| tree | a88bffc862d12a7994d181b3dfa299e9dbfcaf23 /arch/x86 | |
| parent | bf10bd0be53282183f374af23577b18b5fbf7801 (diff) | |
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>
Diffstat (limited to 'arch/x86')
| -rw-r--r-- | arch/x86/kvm/x86.c | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 29d9b078ce69..3b92db412335 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -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) |