[PATCH v4 23/30] KVM: x86: Factor out kvm_use_master_clock()

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

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 used *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@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
---
 arch/x86/kvm/x86.c | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d36d03b8268e..0656d901fe79 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2640,6 +2640,27 @@ 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_freq &&
+		!ka->backwards_tsc_observed &&
+		!ka->boot_vcpu_runs_old_kvmclock;
+}
+
 static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
 {
 #ifdef CONFIG_X86_64
@@ -2662,7 +2683,7 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation)
 	 * are fine — each vCPU's pvclock has its own tsc_timestamp that
 	 * accounts for its offset.
 	 */
-	bool use_master_clock = ka->all_vcpus_matched_freq &&
+	bool use_master_clock = kvm_use_master_clock(vcpu->kvm) &&
 				gtod_is_based_on_tsc(gtod->clock.vclock_mode);
 
 	/*
@@ -3190,10 +3211,9 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 #ifdef CONFIG_X86_64
 	struct kvm_arch *ka = &kvm->arch;
 	int vclock_mode;
-	bool host_tsc_clocksource, vcpus_matched;
+	bool host_tsc_clocksource;
 
 	lockdep_assert_held(&kvm->arch.tsc_write_lock);
-	vcpus_matched = ka->all_vcpus_matched_freq;
 
 	/*
 	 * If the host uses TSC clock, then passthrough TSC as stable
@@ -3203,9 +3223,8 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 					&ka->master_kernel_ns,
 					&ka->master_cycle_now);
 
-	ka->use_master_clock = host_tsc_clocksource && vcpus_matched
-				&& !ka->backwards_tsc_observed
-				&& !ka->boot_vcpu_runs_old_kvmclock;
+	ka->use_master_clock = host_tsc_clocksource &&
+				kvm_use_master_clock(kvm);
 
 	if (ka->use_master_clock) {
 		u64 tsc_hz;
@@ -3231,7 +3250,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
 
 	vclock_mode = pvclock_gtod_data.clock.vclock_mode;
 	trace_kvm_update_master_clock(ka->use_master_clock, vclock_mode,
-					vcpus_matched);
+					ka->all_vcpus_matched_freq);
 #endif
 }
 
-- 
2.51.0