From ab3a60ea68feba49c0d6cbd36cda561205c57f1a Mon Sep 17 00:00:00 2001 From: David Woodhouse Date: Sat, 27 Apr 2024 11:42:53 +0100 Subject: [PATCH] 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 Reviewed-by: Paul Durrant --- 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 7924ab5280af6..f7d9f858dee26 100644 --- 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 -- 2.50.1