[RFC PATCH v3 16/21] KVM: x86: Factor out kvm_use_master_clock()

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Both kvm_track_tsc_matching() and pvclock_update_vm_gtod_copy() make a
decision about whether the KVM clock should be in master clock mode.

They use *different* criteria for the decision though. This isn't really
a problem; it only has the potential to cause unnecessary invocations of
KVM_REQ_MASTERCLOCK_UPDATE if the masterclock was disabled due to TSC
going backwards, or the guest using the old MSR. But it isn't pretty.

Factor the decision out to a single function. And document the historical
reason why it's disabled for guests that use the old MSR_KVM_SYSTEM_TIME.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e21b8c075bf6..437412b36cae 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2518,6 +2518,27 @@ static inline bool gtod_is_based_on_tsc(int mode)
 }
 #endif
 
+static bool kvm_use_master_clock(struct kvm *kvm)
+{
+	struct kvm_arch *ka = &kvm->arch;
+
+	/*
+	 * The 'old kvmclock' check is a workaround (from 2015) for a
+	 * SUSE 2.6.16 kernel that didn't boot if the system_time in
+	 * its kvmclock was too far behind the current time. So the
+	 * mode of just setting the reference point and allowing time
+	 * to proceed linearly from there makes it fail to boot.
+	 * Despite that being kind of the *point* of the way the clock
+	 * is exposed to the guest. By coincidence, the offending
+	 * kernels used the old MSR_KVM_SYSTEM_TIME, which was moved
+	 * only because it resided in the wrong number range. So the
+	 * workaround is activated for *all* guests using the old MSR.
+	 */
+	return ka->all_vcpus_matched_tsc &&
+		!ka->backwards_tsc_observed &&
+		!ka->boot_vcpu_runs_old_kvmclock;
+}
+
 static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
 {
 #ifdef CONFIG_X86_64
@@ -2550,7 +2571,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
 	 * To use the masterclock, the host clocksource must be based on TSC
 	 * and all vCPUs must have matching TSC frequencies.
 	 */
-	bool use_master_clock = ka->all_vcpus_matched_tsc &&
+	bool use_master_clock = kvm_use_master_clock(vcpu->kvm) &&
 				gtod_is_based_on_tsc(gtod->clock.vclock_mode);
 
 	/*
@@ -3096,9 +3117,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 					&ka->master_cycle_now);
 
 	ka->use_master_clock = host_tsc_clocksource
-				&& ka->all_vcpus_matched_tsc
-				&& !ka->backwards_tsc_observed
-				&& !ka->boot_vcpu_runs_old_kvmclock;
+				&& kvm_use_master_clock(kvm);
 
 	/*
 	 * When TSC scaling is in use (which can thankfully only happen
-- 
2.44.0