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

From: Marcelo Tosatti <mtosatti@redhat.com>
To: kvm-devel <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: [PATCH] KVM: x86: allow TSC to differ by NTP correction bounds without TSC scaling
Date: Mon, 15 Jun 2020 08:59:53 -0300	[thread overview]
Message-ID: <20200615115952.GA224592@fuller.cnet> (raw)

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.

Allow a conservative 250ppm error between host TSC and VM TSC frequencies,
rather than requiring an exact match. NTP daemon in the guest can
correct this difference.

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

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3156e25..39a6664 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1772,6 +1772,8 @@ static int set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale)
 
 	/* TSC scaling supported? */
 	if (!kvm_has_tsc_control) {
+		if (!scale)
+			return 0;
 		if (user_tsc_khz > tsc_khz) {
 			vcpu->arch.tsc_catchup = 1;
 			vcpu->arch.tsc_always_catchup = 1;
@@ -4473,7 +4475,8 @@ 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)

