KVM: x86: Factor out kvm_use_master_clock()

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@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7924ab5280af6937298d02a13aff49988860f32c..f7d9f858dee26dee65c84357c4f9bfdb459b5b1d 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2495,6 +2495,27 @@ 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
@@ -2527,7 +2548,7 @@ 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);
 
        /*
@@ -3073,9 +3094,7 @@ 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