On 4/27/2024 4:05 AM, David Woodhouse wrote:
> From: David Woodhouse <[email protected]>
>
> 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 <[email protected]>
> ---
> 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)
typo: 'strut' -> 'struct'
> +{
> + 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) &&
'kvm' should be `vcpu->kvm`
> gtod_is_based_on_tsc(gtod->clock.vclock_mode);
