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 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 --- 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 d6e4469f531a..680b39f17851 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(strut 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(kvm) && gtod_is_based_on_tsc(gtod->clock.vclock_mode);
/* @@ -3089,9 +3110,7 @@ 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