Re: [PATCH] x86/kvm: fix condition to update kvm master clocks

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Jun 13, 2016 at 08:19:30PM +0300, Roman Kagan wrote:
> While we're at this:
> 
> According to the comments in the code, the purpose of the masterclock
> scheme is to prevent any vCPU from seeing an outdated hv_clock of
> another vCPU.

It prevents two vcpus from using different hv_clocks:

"
 * To avoid that problem, do not allow visibility of distinct
 * system_timestamp/tsc_timestamp values simultaneously: use a master
 * copy of host monotonic time values. Update that master copy
 * in lockstep.
"

> However I'm missing how that is achieved.  AFAICS the guest entry is
> allowed as soon as all vCPUs are kicked from guest with
> KVM_REQ_CLOCK_UPDATE set; what stops one vCPU from processing it and
> entering the guest before another vCPU even started updating its
> hv_clock?

static void kvm_gen_update_masterclock(struct kvm *kvm)
{
#ifdef CONFIG_X86_64
        int i;
        struct kvm_vcpu *vcpu;
        struct kvm_arch *ka = &kvm->arch;

        spin_lock(&ka->pvclock_gtod_sync_lock);
        kvm_make_mclock_inprogress_request(kvm);
        /* no guest entries from this point */
        pvclock_update_vm_gtod_copy(kvm);

        kvm_for_each_vcpu(i, vcpu, kvm)
                kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);

        /* guest entries allowed */
        kvm_for_each_vcpu(i, vcpu, kvm)
                clear_bit(KVM_REQ_MCLOCK_INPROGRESS, &vcpu->requests);

        spin_unlock(&ka->pvclock_gtod_sync_lock);
#endif
}